Hladno preoblikovanje konti litega jekla Cold Working of Continuously Čast Steel
B. Arzenšek*1, A. Rodič*2, J. Žvokelj*2
UDK: 621.74.047-426:621.778.011:669.15-194.56 ASM/SLA: Q23q, 1—67, STb, D9q, 4—61
Postopek konti ulivanja tankih profiiov jekel in drugih zlitin je novejši način izdelave predvsem specialnih in slabo preoblikovalnih materialov. Postopek bo že v kratkem uveden na Metalurškem inštitutu, v bodoče pa verjetno tudi v slovenskih železarnah.
Jeklo ima v litem stanju slabe preoblikovalne sposobnosti. V raziskavi smo ugotavljali vlečne sposobnosti konti lite avstenitne nerjavne žice. Cilj raziskave je bil ugotoviti največje deformacije, ki jih lito jeklo pri vlečenju prenese, in vpliv deformacije ter temperature na rekristaiizacijo litega jekla.
1.	UVOD
Postopek konti ulivanja žice je iz tehnološkega in ekonomskega stališča zelo primeren in perspektiven način izdelave tankih profilov jekel in zlitin. Iz tehnološkega zato, ker vsa vroča predelava odpade, iz ekonomskega pa, ker je konti lita žica precej cenejša od klasično izdelane. Pri postopku konti ulivanja ima jeklo oziroma žica lito mikrostrukturo, ki ima precej slabše preoblikovalne sposobnosti kot klasično izdelana, vroče valjana žica. Ker moramo tako žico zaradi nadaljnje predelave v hladnem največkrat še naprej vleči do tanjših dimenzij, smo v raziskavi ugotavljali njene preoblikovalne sposobnosti. Preizkušali smo uvoženo konti lito avste-nitno nerjavno žico, vrste AISI 304, premera 8.1 mm, ki je v odstotkih vsebovala: 0,03 C, 19,1 Cr, 9,2 Ni, 0,14 Nb in 0.13 N. Preoblikovalne sposobnosti jekla smo ugotavljali s krivuljami tečenja. s preizkusi vlečenja in metalograf-skimi preiskavami.
2.	MIKROSTRUKTURNE IN PREOBLIKOVALNE LASTNOSTI KONTI LITE ŽICE
Konti lita žica se razlikuje od klasično izdelane valjane žice po stanju površine žice, po mikrostrukturi, preoblikovalnih lastnostih jekla in poroznosti sredine litega jekla.
2.1. Površina konti lite žice
Na površini konti lite žice so vidne drobne vdolbine, podobne razpokam (slika 1), ki nastanejo pri strjevanju jekla na koncu kristalizatorja. Medsebojna oddaljenost
*1 Boris Arzenšek. dipl. ing. met., Metalurški inštitut Ljubljana, Lepi pot 11, 61000 Ljubljana
*2 SZ-Metalurški inštitut Ljubljana
Continuous casting of thin sectional steels and other alloys is a modern production process primarily for spe-cial materials vvith poor deformability. This process will soon be appiied at the Institute of Metailurgy and prob-ably also in Slovene steelvvorks in the near future.
Because the cold deformability of čast steel is limited the dravving capability of continuously čast stain-less steel vvire vvith an austenitic microstructure vvas examined. The aim of this vvas to determine the maxi-mum strain achievable in dravving čast steel as vvell as the influence of strain and temperature on its recrystaili-zation.
1. INTRODUCTION
From the technological and economic point of vievv the production of continuously čast vvire is a very suit-able and prospective way of manufacturing thin section-als steels and alloys. From the technological point of vievv it is suitable because hot vvorking becomes unne-cessary, and from the economic point of vievv, because vvire čast continuously is considerably cheaper than vvire produced conventionally. Steel, e. g vvire produced by continuous casting. has a microstructure vvith essen-tially vvorse deformability than conventionally manufac-tured hot-roiled vvire. As continuously čast vvire must be additionally dravvn to smaller diameters for further cold vvorking. vvire deformability vvas examined. Imported continuousiy čast vvire from austenitic stainless AISI 304 steel vvith 0.03 % C, 19.1 % Cr, 9.2 % Ni, 0,14 % Nb and 0.13 % N vvith a diameter of 8.1 mm vvas tested. The deformabHity of the steel vvas determined by flovv curves, dravving tests and by metailographic research.
2. MICROSTRUCTURE AND DEFORMABILITY OF CONTINUOUSLY ČAST WIRE
Continuousiy čast vvire differs from conventionally made roiied vvire in vvire surface quaiity, microstructure, deformability and porosity of the čast core.
2.1 The Surface of Continuously Čast Wire
Shallovv circumferentiai marks are visible on the surface of continuously čast vvire (Fig. 1). These marks, vvhich look like fissures, appear vvhen steel solidifies at the end of the mould. The distance betvveen them depends upon the stroke (frequence) of the dravving device. The circumferentiai marks are deeper vvhen using older equipment vvith iovver frequences. vvhereas these marks could completely disappear by using new
Slika 1
Površina konti lite žice Fig. 1:
Continuousiy čast wire surface
Slika 2:
Makroposnetek konti lite žice po litje^pov. 10 x) Fig. 2:
Macrostructure of continuously čast steel (magn. 10x )
Slika 3:
Mikroposnetek konti lite žice po litju (pov. 50 x ) Fig. 3:
Microstructure of continuously čast steel (magn. 50 x )
vdolbin je odvisna od koraka (frekvence) vlečnega mehanizma. Pri starejših napravah z manjšimi frekvencami so vdolbine večje, pri novejših, tudi pri napravi, ki bo že v kratkem pričela redno obratovati na Metalurškem inštitutu, pa pri večjih frekvencah vdolbine lahko popolnoma odpravimo (2,3).
2.2. Mikrostrukturne lastnosti
Mikrostruktura konti lite žice je sestavljena iz zelo tanke plasti globulitnih kristalov in transkiristalov, ki so
Slika 4:
Napetosti tečenja konti litega jekla AISI 304 pred gašenjem in po njem (KL-konti lito, G-gašeno) Fig. 4:
Fiow stress curves of continuously čast AISI 304 steel before and after annealing (KL-continuously čast. G-anneaied)
equipment vvifh higher freguences. like that vvhich will soon be operating at the Institute of Metallurgy (2.3).
2.2 Characteristic of Microstructure
The microstructure of continuously čast wire con-sists of a very thin layer of globulitic crystal grains and columnar crystals orientated towards the centre of the vvire (Fig. 2,3) vvhich can be partly porous. Hovvever. this porosity is totally eliminated by further vvorking (1).
2.3 Deformability of Continuously Čast Wire
Deformability of continuously čast vvire vvas specified by yield stress curves obtained by the compression of test pieces made from continuously čast vvire (Fig. 4) Čast vvire as well as previously quenched vvire vvere tested in order to determine the necessity of quenching ovving to improved deformability. The highest linear strains of continuously čast vvire are betvveen 40 and 45 %, vvhich is approximately haif the strain obtainable in conventionally made vvire vvith a similar chemical composition. Continuously čast and quenched vvire vvithstand a strain vvhich is only a fevv percentage points higher than that of unquenched vvire. vvhich means that quenching after continuous casting is not essential to improve deformability. The lovv reduction of area vvhich vvas observed during the tension test of continuously čast vvire. also confirms the bad deformability of such vvire. The reduction in area vvas only 15 %, vvhich is consider-ably lower than that of conventionally made vvire vvith a similar chemical composition vvhich gives a reduction in area of over 60 %.
2.4 Dravving of Continuously Čast Wire
Dravving tests vvere made on about 2 m long vvire pieces. vvhich vvere dravvn from one coil to another. Before dravving. the vvire surface vvas covered vvith Teh-nolin and Stearat povvder. vvhich is a standard lubricant used in dravving stainless steels. The velocity of dravving vvas 0.24 m/s and the dravving reduction vvas 20 to 25 %. Continuousiy čast vvire vvas dravvn to the highest possible strain. i. e. until the vvire split. Besides the dravving capacity of steel established in the tension test. the dis-
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0.2 0.4 0.6 0.8 1.0 Logoritmična deformacija. True strain.
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usmerjeni pravokotno na središčnico (slika 2 in 3). Sredina konti lite žice je lahko delno porozna, ki pa jo z nadaljnjo predelavo popolnoma odpravimo (1).
2.3.	Preoblikovainost konti lite žice
Preoblikovalne sposobnosti konti lite žice smo najprej ugotavljali s krivuljami napetosti tečenja, dobljenimi s stiskanjem preizkušancev, izdelanih iz konti lite žice (slika 4). Preizkušali smo žico v litem stanju brez gašenja in po gašenju z namenom, da bi ugotovili, ali je zaradi boljše predelovalnosti žico po litju potrebno gasiti ali ne. Največje specifične deformacije, ki jih konti lita žica prenese, znašajo le 40 do 45 %, kar je približno dvakrat manj kot pri klasično izdelani žici podobne kemične sestave. Gašena konti lita žica prenese le nekaj odstotkov večje deformacije kot negašena, kar pomeni, da žice po konti litju za boljšo preoblikovainost ni potrebno gasiti. O slabih preoblikovalnih lastnostih preizkušane konti lite žice lahko sklepamo tudi iz nizke kontrakcije, ki smo jo dobili pri trgalnem preizkusu žice. Kontrakcija je znašala le 15 %, kar je precej manj kot pri klasično izdelani žici podobne kemične sestave, kjer znaša več kot 60 %.
2.4.	Vlečenje konti lite žice
Preizkuse vlečenja smo naredili na približno dva metra dolgih koncih žice in z vlečenjem iz kolobarja v kolobar. Pred vlečenjem smo na površino žice nanesli Tehnolin in stearatni prašek, standardno mazivno prevleko za vlečenje nerjavnih jekel. Hitrost vlečenja je bila 0,24 m/s, redukcije pa so znašale od 20 do 25 %. Konti lito žico smo vlekli do največjih možnih deformacij, do trganja žice. Med vlečenjem smo poleg ugotavljanja vlečnih sposobnosti jekla, spremljali tudi zapolnjevanje vdolbin na površini žice in zapolnjevanje mikroporozno-sti na sredini žice.
Največje deformacije, ki smo jih pri vlečenju iz kolobarja v kolobar dosegli, so znašale približno 35 % (pri vlečenju do premera 6,5 mm) in približno 45 % pri vlečenju krajših koncev žice. Dosegli smo jih pri dvakratnem vlečenju žice. Deformacije, ki smo jih pri vlečenju dosegli, so približno enake deformacijam, doseženim pri stiskanju jekla. Po vlečenju .smo žico žarili pri temperaturi 1050° C in gasili. Pri omenjenem žarjenju je jeklo rekri-staliziralo. O rekristalizaciji konti litega vlečenega jekla bomo podrobneje spregovorili v nadaljevanju. Rekristali-zirano žico smo nadalje vlekli pri enakih redukcijah, velikih 20 do 25 %. Vlekli smo jo v štirih vlekih iz premera 6,1 do 3,5 mm. Skupna deformacija je znašala 86%, kar je deformacija velikostnega reda vlečenja klasično izdelane avstenitne nerjavne žice, zato preizkušane žice nismo več vlekli do tanjših dimenzij. Iz omenjenih rezultatov lahko zaključimo, da ima konti lita žica po vlečenju in rekristalizacijskem žarjenju podobne vlečne sposobnosti kot klasično izdelana žica podobne kemične sestave.
Globina vdolbin, nastalih med litjem žice, se je med vlečenjem zmanjševala tako, da jih pri premeru vlečene žice 5,2 mm, pri redukciji žice e = 60 %, pri metalografs-kem pregledu površine žice nismo več opazili. Podobno je bilo tudi z mikroporoznostjo jekla, ki je pri premeru vlečene žice 5,2 mm, pri zmanjšanju preseka žice približno za polovico, nismo več opazili.
3. REKRISTALIZACIJA KONTI LITEGA HLADNO VLEČENEGA JEKLA
Rekristalizacija konti litega hladno vlečenega jekla je odvisna od stopnje deformacije in višine temperature žarjenja. Ker konti lito jeklo prenese precej manjše
appearance of marks on the wire surface as well as the fillings of microporosity in the wire core were also deter-mined. The highest strain attainabie whiie dravving from one coii to another was approximateiy 35 % (in dravving to a diameter of 6.5 mm) and approximateiy 45% in dravving shorter pieces of vvire. Such strains vvere obtained by pas s ing the vvire tvvice. The strains obtained by dravving vvere simiiar to the ones obtained in steel compression tests. After dravving, the vvire was anneaied at 1050° C and then quenched. During annealing the steel recrystallized. Recrystallization of continuously čast steel will be treated more in detaii The recrystal-lized vvire was further dravvn in four passes vvith nearly the same dravving reduction of about 20 to 25 % from a diameter of 6.1 to 3.5 mm. The total dravving reduction was 86 %, vvhich is the same order of magnitude as for conventionally made stainiess steel vvire. This is the rea-son why the tested vvire vvas not further dravvn to a smaller diameter.
On the basis of these results it could be concluded, that after dravving and recrystallization annealing, contin-uously čast vvire has the same dravving abiiity as conven-tionally made vvire vvith a simiiar chemical composition.
The depth of circumferentiai marks vvhich appeared during casting decreased so much during dravving that the marks vvere no longer visible at metallographic examination of the vvire (reduced by e= 60 % from a diameter of 5.2 mm). Similarly, the microporosity of the steel vvas no longer observed vvhen the cross-section of the vvire vvith a diameter of 5.2 mm vvas reduced approxi-mateiy by one haif.
3.	RECRYSTALLIZATION OF CONTINUOUSLY ČAST COLD-DRAWN STEEL
Recrystallization of continuously čast cold rolled steel depends upon the degree of deformation and the annealing temperature. As continuously čast steel vvith-stands considerably lovver strains than conventionaily made steel. the purpose of this investigation vvas to estabiished the sma/lest strains and the lovvest temperatures at vvhich steel completely recrystallizes. The degree of recrystallization vvas determined by metallographic observations and hardness measurements. The recrystallization of steel vvas estabiished at tempere-tures from 900 to 1050° C and during a dravving reduction of 9 to 44 %. The most typicai microstructures Of steel after recrystallization annealing are shovvn in Fig. 5 At 900°C steel did not recrystaiiize until a reduction of 44 % vvas achieved, vvhereas at temperatures of 1000 and 1050° C steel recrystallized already at a reduction of 9 %. Steel dravvn after recrystallization recrystallizes at the same temperatures and strains as constinuously čast dravvn steel.
4.	C ON C L USIONS
The dravving abiiity of continuousiy čast AISI 304 stainiess steel vvire vvith an austenitic microstructure vvas examined. The aim of this vvas to estabiish its dravving abiiity vvhen čast and after recrystallization anealing, as vvell as the influence of surface conditions and micro-porosity of the vvire core on its dravving abiiity and its recrystallization properties. To summarize:
1.	Before dravving for better workability it is not necessary to anneai vvires from austenitic AISI 304 stainiess steel produced by continuous casting process.
2.	The total strain of continuousiy čast steel vvhiie dravving reaches 35 to 45 %, i. e. approximately less than half that of conventionally made steel.
deformacije kot klasično izdelano jeklo, smo v raziskavi želeli ugotoviti tiste najmanjše deformacije in najnižje temperature, ko jeklo še popolnoma rekristalizira. Stopnjo rekristalizacije smo ugotavljali z metalografskimi preiskavami in meritvami trdot. Rekristalizacijo jekla smo ugotavljali pri temperaturah žarjenja od 900 do 1050° C in redukcijah od 9 do 44 %. Najznačilnejše mikrostrukture jekla po rekristalizacijskem žarjenju so prikazane na sliki 5. Pri temperaturi 900° C je jeklo rekristaliziralo šele pri stopnji deformacije 44%, pri temperaturah 1000 in 1050° C pa že pri stopnji deformacije 9 %. Jeklo, vlečeno po rekristalizaciji, rekristalizira pri enakih temperaturah in deformacijah kot konti lito vlečeno jeklo.
4. ZAKLJUČKI
V raziskavi smo ugotavljali vlečne sposobnosti konti lite avstenitne nerjavne žice, vrste AISI304, ulite na napravi za konti litje žice. Cilj raziskave je bil ugotoviti njegove vlečne sposobnosti v litem stanju in po rekristalizacijskem žarjenju, vpliv stanja površine in mikroporoz-nosti sredine žice na njegove vlečne sposobnosti in ugotoviti njegove rekristalizacijske lastnosti. Kratki zaključki so naslednji:
1.	Žice iz avstenitnega nerjavnega jekla AISI 304, izdelane po postopku konti ulivanja, pred vlečenjem za boljšo preoblikovalnost ni potrebno gasiti.
2.	Preizkušano konti ulito jeklo prenese pri vlečenju 35 do 45 % skupne deformacije, kar je približno polovico manj kot pri klasično izdelanem jeklu.
3.	Rekristalizirano konti lito jeklo ima podobne preoblikovalne sposobnosti kot klasično izdelano jeklo.
4.	Deformirana lita mikrostruktura jekla rekristalizira pri temperaturi žarjenja 900° C pri specifični deformaciji približno 44%, pri temperaturi 1000° C pa že pri specifični deformaciji približno 9 %. Podobno je tudi pri rekristalizirani deformirani mikrostrukturi.
5.	Vdolbine, ki nastanejo pri postopku konti ulivanja na površini žice, ne vplivajo na vlečne sposobnosti preiz-kušanega jekla. Vdolbine in mikroporoznost jekla odpravimo s predelavo jekla na polovico preseka konti litega jekla.
Preizkušano konti lito jeklo je bilo legirano z dušikom, saj ga je vsebovalo kar desetkrat več, kot ga vsebuje navadno jeklo AISI 304. Dušik dodajamo v jeklo za povečanje njegove trdnosti in obrabne obstojnosti. S tem zmanjšamo njegove preoblikovalne sposobnosti, zato lahko pričakujemo, da ima konti lito jeklo brez legi-
3.	Recrystatlized continuousty čast steel has a simi-lar workablllty as conventionally made steel.
4.	The microstructure of čast and deformed steel recrystallizes at an anneallng temperature of 900" C and a linear strah of about 44 %, whereas at a temperature of 1000° C. It already recrystalllzes at a linear strain of about 9 %. This is also true for the microstructure of the steel which has been deformed after recrystailization.
5.	Marks which appear on the surface of continuously čast wire do not affect a) the dravving ability of the tested steel and b) the microporosity of the core of the continuously čast wire. Marks and microporosity are eii-minated by vvorking čast steel reduced to haif its initial cross-section.
The continuously čast steel was alloyed with nitrogen. In fact. it contained 10 times more nitrogen
Slika 5:
Mikrostrukture jekla AISI 304:
a)	vlečenega pri stopnji deformacije 26 % in žarjenega pri temperaturi 900° C, nerekristalizirano;
b)	vlečenega pri stopnji deformacije 44 % in žarjenega pri temperaturi 900°C, rekristalizirano;
c)	vlečenega pri stopnji deformacije 9 % in žarjenega pri temperaturi 1000° C, rekristalizirano
Fig. 5:
Microstructure of AISI 304 steel:
a)	reduced by 26 % at dravving and annealed at 900° C. unrec-rystaliized.
b)	reduced by 44 % at drawing and annealed at 900° C, recrys-tall/zed.
c)	reduced by 9 % dravving and annealed at 1000° C, recrystal-iized
ranega dušika boljše preoblikovalne sposobnosti. To nam potrjujejo tudi mehanski preizkusi in preizkusi vlečenja prvih vzorcev konti ulite žice iz jekla AISI 304 (z normalno vsebnostjo dušika), ulitih na napravi na Metalurškem inštitutu. Kontrakcija žice je znašala 37 %, največja deformacija, ki smo jo pri vlečenju krajših koncev žic dosegli, pa 56 %.
than ordinary AISI 304 steel. Nitrogen is added to steel in order to increase its strength and resistance to wear. In doing so the workability of steel is decreased, and that is why a better workabiiity of continuously čast steel without alloyed nitrogen can be expected. This has also been confirmed by mechanical tests and drawing tests made on the first specimens of continuously čast wire from AISI 304 steel (with a normal content of nitrogen), vvhich were čast at the Institute of Metallurgy. The area reduction during the vvire tension test was 37 % — the highest deformation achieved in dravving shorter pieces of vvire vvas 56 %.
LITERATURA / REFERENCES
1. B Arzenšek, A. Rodič, J. Žvokelj, N. Vojnovič, D. Lazar: 2. Haissig M.: Horizontal continuous casting - recent deve-Hladno vlečenje in izdelava vijakov iz uvožene konti lite ne- lopments and future trends. Steel times 1986 10 546-548 rjavne žice, Poročilo Ml, 1987.	3 j Rodič: Skrajševanje tehnološkega postopka od taline do
žice, ŽEZB22, 1988, 4, 101 — 109.