SCIENCERESEARCHDEVELOPMENT

SCIENCE*RESEARCH*DEVELOPMENT

Nenad Redovic, Dorde Drobnjak

RAZVOJ CEUKA ZA IZRADU ZAVARENIH KONSTRUKCIJA POVISENE SIGURNOSTI DEVELOPMENT OF STEELS FOR FABRICATION OF WELDED CONSTRUCTIONS WITH IMPROVED SAFETY

Rad primljen I Paper received: 04.09.2001

Adresa autora I Author's addres: Dr Nenad Radovi6, prof. dr Dorde Drobnjak Tehnolosko-rnetalurski fakultet - Katedra za fizicku metalurgiju Karnegijeva 4, 11120 Beograd email: nenrad!d~!ll!2.JJnfbgac.Yl!

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

Keywords: Steels, Structural Steels, Development, Thermomechanical Treatment, Reliability, History, Reviews, Future

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

Izvod U radu su izloiene tisiko-rnetslurske osnove istorijskog razvoja celicnih materijala, primeri savremenih roseate celik« za speciticne namene i pretpostavke da/jeg razvoja. Nedvosmisteno, u buducnosti, ceticn! materijali ce ostati suvereno najrasprostranjeniji konstrukcioni materijal, zbog najbo/jeg odnosa kvalitet/cena. Dominantno mesto ce zauzimati celic! predvideni za izradu zavarenih konstrukcija, a njihov razvoj ce biti omoqucen konceptom totalne termorneherucke prerade. Uporedo, predvide se intenzivan razvoj na polju tehnologija zavarivanja, dodatnih materijala, metemetickoq modeliranja termomehenicke prerade i zavarivanja, sa ci/jem da se obezbedi izrada zavarenih konstrukcija povisene sigurnosti.

Abstract Physical and metallurgical basis of historical steel development and some modern solutions for specific applications, together with predictions and projections of future development are reviewed in this paper. Undoubtedly, in future, steel will remain absolutely most used construction material due to the best relationship properties/price. Dominant place is reserved for steels for welded constructions, and their further development will be provided by application of concept of Total Thermomechanical Control Processing (TTCMP). Simultaneosiy, predictions consider intensiv development in the field of welding procedures, Itller materials, modelling of TMCP and welding, with aim to provide fabrication of welded constructions with improved safety.

UVOD

INTRODUCTION

Procene ukazuju da 6e industrija do 2020. godine, nevezano za razlog, zahtevati promenu iii alternativu za oko 95 % materijala iz danasnje industrijske upotrebe [1]. ave projekcije se moraju uzeti sa rezervom, iz iskustva 0 projekcijama zamene celika drugim konstrukcionim materijalima, koje su radene 60-ih godina prosloq veka, a koje su predvidale dramaticno smanjenje upotrebe celika na racun obojenih metala, sto se nije obistinilo ni u malom procentu [2]. Razlog poqresnih procena je superiornost celika u pogledu odnosa zahteva kvaliteta i cene za koju se taj kvalitet obezbeduje. Na razvoj celika u proteklih vise od 100 godina su paralelno uticala dva pravca: razvoj i osvajanje znanja iz fizicke metalurgije i razvoj opreme i tehnologije za izradu celika.

Today's predictions suggest that in year 2020, about 95 % of present industrial materials will be replaced or modified [1]. On the other hand, about 40 years ago, the similar types of predictions have predicted dramatically decrease in steel use on behalf of nonferrous metals. Since those predictions were completely wrong, these types of predictions have to be taken into account very precociously [2]. The main reason for failing in predictions is the steel superiority in properties to price ratio in comparison to other metals. Two major influences on steel development in last more than 100 years were: development in knowledge in the fields of physical metallurgy and development in steel processing equipment and technologies

TOK RAZVOJA KONBTRUKCIONIH CELIKA

DEVELOPMENT OF STRUCTURAL STEELS

Kod obicnih ugljenicnih celika, kao prve i najstarije vrste cellka koja je proizvedena, povecan]e cvrstoce je zasnivano na pove6anju sadrzaja C koji je u Fe cqraniceno intersticijski rastvoren. U toku deformacije, kao posledica primene spoljnog naprezanja, dislokacije pri kretanju nailaze na prepreke, sto zahteva povecan]e

First and the oldest type of steels used as structural steels are plain carbon steels. The increase in strength 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 with obstacles, what in turn requires increase of applied

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SCIENCE*RESEARCH*DEVELOPMENT naprezanja za dalju deformaciju. Kako je dominantna uloga rastvorenih atoma ugljenika, ovaj mehanizam se naziva rastvarajuce ojacavanie Ovi celici, pored C, sadrzali su i znatne koliCine Si, S, P, i u nekim slucajevima Mn (iskljucivo zavisno od sadrzaja Mn u rudi), i u tim vremenima su predstavljali znatan napredak i omoguGili znacajno prosiren]e korlscenja celika kao konstrukcionog materijala. Struktura je zavisila od sadrzaja C i bila je uglavnom feritna, feritnoperlitna, a veoma retko beinitna. Kriticna primena ovih celka je bila vezana i za prisustvo SiP, koji se kao necistoce, zbog niskog nivoa tehnologije, nisu mogli uspesno odstraniti pri proizvodnji celika. Sumpor sa zelezorn gradi niskotopivi eutektikum, koji se u obliku tankog filma rasporeduje po granicama zrna, te dovodi do lorna. Generalno, povecan]e cvrstoce ugljenicnih celika izazvano povecanjern sadrzaja C nije moglo da resi: zahtev za povisenu zilavost i nisku prelaznu temperaturu; zavarljivost i velike tezine konstrukcije. Sa povecanjern sadrza]a C preko 0.8 % u mikrostrukturi dolazi do izdvajanja cementita, koji pri zagrevanju rnoze da formira karbidnu rnrezu sa cementitnim lamelama veoma ostrih ivica, ko]e predstavljaju koncentratore naprezanja i smanjuju zilavost.

strength for further deformation. Dominant role in strengthening is attributed to carbon mechanism is named solution strengthening. Besides carbon, these steels contented respectable amounts of Si, S, P and Mn in some cases (depending on Mn content in iron ore). They provided respectable broadening of use of steel as material for constructions. In these steels, microstructure depended on carbon content, and was mainly ferritic or ferritic-pearlitic, and in some cases evens bainitic. Critical property that have limited application of this steels was the content of impurities, mainly Sand P. Due to low technology level, Sand P it was not possible to be removed during processing. Sulfur forms a lowmelting eutecticum with Fe, situated as a thin film on grain boundaries, leading to fracture. Generally speaking, increase in strength in plain C steels due only to increase in carbon content could not deal with three major problems: improved toughness and low transition temperature, weldability and enormous weight of construction itself. Also, in steels with more than 0.8 % C, cementite is introduced, and can in some cases lead to formation of carbide net characterized with very sharp edges which behave as stress concentrators decreasing toughness.

C-Mn celici

C-Mn Steels

Za komponente konstrukcije koje zahtevaju pobollsane rnehanicke osobine, zahtev bolje zilavosti moze da ispuni samo celik sa nizlrn sadrzajern C, ali je njegova cvrstoca niza, Zato je potreban veci presek konstrukcije, sto povecava njenu tezinu. PrevazilaZenje problema je bilo rnoquce tek razvojem nove vrste cellka, uvodenjem C-Mn celika, Uloga Mn je dvojaka: uticaj na ojacavan]e i kontrola sumpornih ukljucaka, Intezitet rastvarajuceq ojacavanja zavisi od vrste leqirajuceq elementa, odnosno da Ii dodati elemenat stvara supstitucijski iii intersticijski cvrsti rastvor. Mn je u Fe rastvoren supstitucijski, i efekt ojacavanja pre svega zavisi od razlike u vellcinl atoma. Kako su Mn i Fe susedni elementi u periodnom sistemu, sam efekt ojacavania nije toliko znacajan i primaran. Prisustvo Mn je uobicajeno u granicama do 1.5-1.7, i efekt ojacavanja usled Mn se rnoze lako odrediti. Smatra se da u niskouqflenlcnlrn celicima veci dodatak Mn izaziva pojavu proeutektoidnog ferita i tako podize Ar3 temperaturu [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 toku ocvrscavarua. Usled velikog afiniteta Mn i S, najjednostavniji nacin za potpuno vezivanje S je dodatak dovoljne koliCine Mn (u visku). Ova uloga Mn je omoguGila da se u konstrukcionim celicirna znacajno poveca zilavost. U toku dalje prerade MnS se iii izduzuie (valjanje) iii lomi i sitni (kovanje). Prisustvo velikih izduzenlh MnS ukliucaka u valjanim proizvodima je opasno, jer se MnS ponasa kao koncentrator naprezanja, a izaziva i pojavu lamelarnog cepanja.

For construction components which require improved mechanical properties (higher toughness) only low carbon steel can be used, but its strength is relatively low. Therefore, larger cross-sections should be used, what in turn, lead to increase in weight of construction. To overmatch these problems it was necessary to produce a new type of steel, C-Mn steels. Introduction of manganese had two roles: control of sulfur containing inclusions and influence on strengthening. Degree of solid solution strengthening depends on the type of alloying element, i.e. if atoms of alloying elements are soluted in substitutional or interstitial positions in lattice. Mn is substitutionaly soluted in Fe, but the effect on strengthening is not pronounced. since it depends on the difference in atom size and Mn and Fe are neighboring elements in periodic system. In structural steels, the content of Mn is in most cases limited to 1.5-1.7 %, because larger content would lead to 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 way to completely remove S from steel is addition of sufficient amount of Mn. This role of Mn ensured significant rise in toughness. During further metal working, MnS inclusions can become elongated (rolling) or fractured and dispersed (forging). Presence of long elongated MnS inclusions in as rolled steels is very dangerous, since the edges behave as stress concentrators, leading to fracture or to lamellar tearing in welded constructions.

Umirenl celici kao

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

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

NAUKA*ISTRAZIVANJE*RAZVOJ


dovedene u toku izrade celika. Za eliminisanje ove pojave bilo je potrebno dodati neki od hemijskih elemenata koji imaju veliki afinitet prema kiseoniku, a odgovor je dobijen iz periodnog sistema i na osnovu Ricardsonovog dijagrama (dijagram stabilnosti oksida). Najcesce su koriscen' AI i Si, u kollcinl koja se odreduje na osnovu poslednje hemijske analize u toku proizvodnje celika, koja se uzima iz medulonca. Tako je dobijena nova vrsta celika, tzv. umireni celici, Ime su dobili po ponasaniu u toku livenja, jer kako vise nije bilo slobodnog kiseonika nije dolazilo do oksidacije preostalog C i izdvajanja gasovitog produkta CO, pa se na povrsini nisu zapazali rnehurici, vee je povrslna bila mirna. Dodatak AI je postao zanimljiv. jer je primeceno da je u nekim AI-umirenim celicirna doslo do talozen]a aluminijum nitrida po granicama zrna, te je tako granica zrna mehanlcki blokirana. Rec je bila 0 celicima u kojima je AI dodat u velikom visku i koji su imali visok sadrza] N (period u kome je u konvertore uduvavan vazduh, a ne kiseonik), i usled velikog afiniteta AI i N dolazi do talozenja AIN po granicama zrna. To je prvi stuca] empirijske kontrole granica zrna koji nije doziveo vecu ekspanziju, jer tehnicke rnoqucnosti u proizvodnji nisu mogle uvek da obezbede zellenu raspodelu aluminijuma (za umirenje - AI203 iii za talozenie na granicama zrna - AIN), vee je zbog amfotermnosti dolazilo i do stvaranja ukljucaka tipa aluminata, sto je prakticno bio gUbitak alurnlnhuraa.

Killed Steels

Mikrolegirani cetlcl Razvojem fizlcke metalurgije dobijeni su odgovori na mehanizme deformacionog ojacavanja i rekristalizacije. lstrazivaci su pokusavali da u laboratorijskim uslovima u periodnom sistemu pronadu elemente koji ce dati po mehanizmu slicno ponasanje AI, ali da se to ponasanje rnoze bolje kontrolisati. Tako su razvijeni potpuno novi, do tada nepoznati celici, legirani sa Nb, Ti, V, Zr, B i sl. kod kojih dolazi do povecan]a cvrstoce dodatkom male kotictne legirajueeg elementa. Hronoloski razvoj upotrebe pojedinih mikroleqirajucih elemenata u celicima je dat na slici 1 [4]. Dominacija nekog od navedenih elemenata je zavisila od cene i pogodnosti za terrnornebanicku obradu. Posto su se dodavali u farmaceutskim kolicinarna, poznati su i kao mikrolegirani celici. Ovaj pojam se tradicionalno vezivao za nlskouq'[enlcne cellke povlsene cvrstoce koji su sadrZavali male kolicine Nb i/ili V. Zato je najprihvatljtvija definicija da su mikrolegirani celici oni cetici kod kojih mali dodatak legirajueih elemenata dovodi do intenzivnog smanjenja zrna i/ili taloznoq ojacavanja, usled izdvalanla stabilnih cestica karbida, nitrida iii karbonitrida [5-9]. To su cellcl legirani sa Nb i/ili Vi/iii Ti, u ukupnom sadrzaju sva tri elementa ispod 0.15 % Masovna upotreba i razvoj mikrolegiranih cetika se vezuje za pocetak 60-ih godina pros log veka, sa pocecirna komercijalne proizvodnje ferolegura, posebno FeNb [5]. Glavni motivi za njihov razvoj su bili: znacajno povecan]e cvrstoce, a time i smanjenje tezine konstrukcije iii povecan]e nosivosti; rnoqucnost veoma razlicite termomehanicke obrade: potreba svetskog trzista za zavarljivim ceticima povisene cvrstoce za

Next problem that was facing steel producers was a presence of free oxygen, oxygen originated from air blowing in converter or in furnace. To eliminate the presence of oxygen it was necessary to modify chemical composition with some chemical element that has strong afinity to oxygen. The answer came from Richardson's diagram (diagram of stability of oxides). Usually Si or AI has been used in amount that was estimated from the last chemical composition analysis during steel production (ladle after converter). This procedure introduced new type of steel, so called killed steels. They have been named after their behavior during casting. Since there was no free oxygen. remaining carbon was not able to oxidize, and to produce bubbles of CO. Therefore, the surface in ladle was still. Furthermore, addition of AI became more interesting due to some observations that showed that in some AI-killed steels a grain boundaries precipitation of AIN occurred, decreasing grain boundary mobility. This was observed in steels in which AI was added in high amount; air-blowing (instead oxygen nowadays) led to reasonable presence of nitrogen and strong afinity between AI and N. This was first empirical case of grain boundary control. but it was not followed with greater industrial application. Technology at the time was not able to control the distribution of AI (A1203 for oxidation and AIN for precipitation), because, some amount of AI would be lost in inclusions as aluminates. Microalloyed Steels The behavior of AI in killed steels became a major topic in research. On one hand, physical metallurgy have defined mechanisms of deformation strengthening and recrystallization. and on the other hand, it was a great effort to find elements from periodic system that will have behavior similar to AI, but with much better control. First on laboratory scale, and later on full industrial scale a completely new steels have been introduced. alloyed with Nb, Ti, V. Zr, B etc. In these steels. a large increase in strength is due to addition of very small amount of listed elements. Chronological development of the use of microalloying elements in steels is given in Fig.1[4]. "Domination" of any element can be attributed to price and advantages for thermomechanical processing. Since the addition of alloying elements is on the "pharmaceutical" level, these steels are commonly called microalloyed steels. This name was traditionally related to high strength lowcarbon steels alloyed with Nb and/or V. Therefore. it seems that proper definition is that microalloyed steels are steels in which small addition of alloying elements lead to intensive grain refinement and/or precipitation hardening due to precipitation of stable carbides, nitrides or carbonitrides [5-9]. Nowadays. those steels are alloyed with Nb and/or V and/or Ti, in total amount of 0.15%. Massive use and development of microalloyed steels are associated with early '60 in last century, especially with commercial production of ferrous alloys like FeNb [5]. The main motives for developing MA steels were: significant increase in strength. leading to


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SCIENCE*RESEARCH*DEVELOPMENT cevovode, za sta nije bilo mogu6e primeniti klasi6ni recept za pove6anje cvrtoce sa pove6anjem sadrzaja ugljenika i legiraju6ih elemenata. Struktura mikrolegiranih 6elika posle tople plasti6ne prerade je tipicno sitnozrna i sastoji se od feritnih (a) zrna male veli6ine, homogenih po obliku. Prisutna je i mala koli6ina cementita (zbog 6ega se ovi celici cesto nazivaju niskoperlitni) kao i finodispergovane 6estice karbonitrida, koje se mogu identifikovati samo ispitivanjem na elektronskom mikroskopu. U toku zavrsnoq valjanja obezbeduje se pojava velikog broja preferentnih mesta na kojima je favorizovana pojava klica a-faze pri hladenju ispod Ar3 temperature. Preferentna mesta koja smanjuju energetsku barijeru za pojavu klica a-faze su dislokacije, granice zrna i subzrna, dvojnici, deformacione trake itd. Ukupan broj preferentnih mesta je direktna posledica primenjenog postupka prerade i stepena deformacije. Kako je prerada na povisenim temperaturama, u toku prerade su prisutni i procesi obnavljanja deformisane strukture, oporavljanje i rekristalizacija. U toku rekristalizacije deformisana struktura se zamenjuje nedeformisanom, a posledica je smanjenje gustine dislokacija. Dakle, u toku prerade uporedo se odigravaju dva procesa koji su suprotni po svojoj prirodi: pove6anje i smanjenje gustine dislokacija. Mehanizam spre6avanja rekristalizacije je smanjenje brzine nastanka klica i/ili pokretljivosti granica zrna i subzrna, usled prisustva rastvorenih atoma u 6vrstom rastvoru (kontinuirano valjanje - kratke pauze, nema talozen]a: dominantna uloga Nb) iii cestica taloga (reverzivno valjanje duge pauze; dominantno izdvajanje karbonitrida) [6]. TMCP celici Poznavanjem navedenih fenomena, bilo je mogu6e osmisliti potpuno novu tehnologiju tople plasti6ne prerade, koja se zasniva na kontrolisanju mikrostrukture, posto su poznati procesni parametri (stepen redukcije po stanu, brzinu deformacije, temperaturu) i hemijski sastav. Ova tehnologija je nazvana termomehani6ka kontrolisana prerada (Thermo-Mechanical Control Processing - TMCP) iii samo termomehani6ka prerada [7-9]. Shematski prikaz termomehani6ke prerade mikrolegiranih celika je dat na slici 2, i obuhvata nekoliko tehnologija [8]: 1. Rekristalizaciono kontrolisano valjanje (Recrystallization Control-Rolling - RCR). Predvaljanje i zavrsno valjanje se izvode na visim temperaturama, u podru6ju u kome ne dolazi do termi6kog talozenja, te je staticka rekristalizacija izmedu provlaka potpuna, kompletno iznad TNR (temperatura ispod koje je rekristalizacija nepotpuna). Kako je kompletna deformacija u rekristalisanom podrucju, pocetna grubozrna struktura (sl. 2(a)), posle deformacije se sastoji od homogenih rekristalisanih zrna (sl. 2(b)), i hladenjem na vazduhu se dobija homogena feritna struktura (sl. 2(b')). Ova tehologija se koristi za preradu debelih limova, kod kojih nije mogu6e primeniti klasi6no kontrolisano valjanje, posto valiacki stanovi ne mogu da izdrze jako veliki pritisak valjanja.

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either lowering the construction weight or increase in carrying capacity; thermomechanical treatment: A demand on world market for steels with good weld ability for pipelines, in which was not possible to use "traditional" way to increase strength by heavier alloying and carbon content.Microstructure of MA steels after hot working is typically fine-grained and consists of small and homogenous ferrite (a) grains. Small amount of cementite is also present (low pearlite steels is also used), together with fine dispersed carbonitride particles which can be observed only on electron microscope. During finishing hot rolling, a high number of preferable places for nucleation of a grains during cooling below Ar3 temperature are provided. Preferable places (dislocations, grain and subgrain boundaries, deformation twins, deformation bands) decrease energetic barrier for nucleation and their number directly depends on applied thermomechanical regime and overall strain. During hot deformation, two opposite processes occurs simultaneously: increase of dislocation density due to plastic deformation and decrease of dislocation density due to recrystallization. The mechanism of recrystallization is nucleation of nondeformed microstructure and growth. Main mechanism of suppression of recrystallization is decreasing (or even full blockade) the rate of nucleation and/or grain and subgrain boundary mobility, due to presence 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 - Mechanical Control Processing -TMCP. Schematic illustration of TMCP of microalloyed steels is given in Figure 2, and includes several technologies [7-9]: 1. Recrystallization Control Rolling - RCR. Both roughing and finish rolling are performed on high temperatures (above Tnr - temperature below which recrystallization is partial), in temperature range in which full static recrystallization between passes takes place. Since complete deformation is in recrystallization temperature range, starting relatively coarse grains (Fig.2, sketch a), after deformation transforms into


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