The Smelting of Steel with Specified Characteristics of Nonmetallic Inclusions. Review

Transcription

1 1 The Smelting of Steel with Specified Chrcteristics of Nonmetllic Inclusions. Review A. Alexeenko 1, D. Ponomrenko 2 1) Lortory of specil metllurgy Co., 2) Innovtion ureu of metllurgicl technologies Co. Some steel grdes, e.g. ril, cord, deep drwing, pipe steels, etc. re strictly controlled on nonmetllic inclusions chrcteristics (composition, size, quntity per unit). It is necessry to gurntee good workility during mnufcturing nd high mechnicl properties of products. They re, for instnce: - good metl cstility (especilly of Al-killed steels); - rekless wire drwing of cord steels; - defectless cold rolling nd deep drwing of LCAK-, ULC-, IF-steels; - high level of mechnicl properties, contct ftigue resistnce (of ril nd wheel steels), nd pitting corrosion resistnce (of pipeline steels). It is known tht the influence of nonmetllic inclusions on steel properties my e direct nd indirect. Direct detrimentl effect of inclusions is connected with rekge in wire drwing, ppernce of surfce defects during cold rolling or deep drwing, pitting growth, decrese of contct ftigue resistnce, etc. Indirectly, through interconnection with the microstructure, inclusions influence steel mechnicl properties. For exmple, inclusions of certin composition evenly dispersed through the ulk of metl fcilitte grin size reduction nd consequently to increse of strength nd toughness. In some cses, the oxide metllurgy methods re permit to exclude het tretment fter forging or considerle increse strength nd toughness of het ffected zones (HAZ) 1. The oxide inclusion control pursues the tsks: ) to limit mximum inclusion size (it my vry from 15 to 100 µm for different steel grdes) nd ) to ensure presence of microinclusions of required types only nd restricted in numer per unit. Mcroinclusions oserved in crude steel s rule re products of microinclusions cogultion or reoxidtion (fig. 1, 2). Also they my hve exogenous nture, e.g. refrctory or slg prticles (fig. 3). Lrge deoxidtion products re successfully removed during secondry steelmking opertions, therefore they re usully sent. Min mesures to decrese of mcroinclusions contmintion of continuous csting illets or sls re s follows 2-6 : 1) To limit microinclusions mount in csting steel ecuse they hve tendency to cogulte up mcroinclusions due to turulent metl flows during csting; 2) To use lrge volume tundish to increse time of inclusions removl; 3) To optimize tundish geometry nd inner devises for enhnce of flow control; 4) To prevent metl ginst reoxidtion during secondry steelmking nd csting especilly in troule spots, such s the ldle slide gte, joints of the long nozzle nd ldle s well s sumerged entry nozzles nd the tundish, melt surfce in the tundish; 5) To pply sumerged entry nozzles with glze fcing, therml insultion, supply of inert gses to the teeming chnnel. 6) To use snd filler for sliding nozzle providing relile opening of the ldle slide gte with the long nozzle instlled. 7) To use optiml tundish flux to secure the melt surfce from ir oxidtion, nd to ensure good therml insultion nd inclusions sorption; therewith the flux must e neutrl to csting steel; 8) To mintin stedy metl level in the tundish nd to reduce level fluctution in the mold; 9) To mintin stedy flux supply into the mold.

2 2 Microinclusions hve nother origin thn mcroinclusions. They nuclete nd grow in melt during secondry tretment nd csting or during crystlliztion process. Composition of microinclusions usully corresponds to tht of the metl. To mnufcture steel with non-metllic inclusions with specified chrcteristics, it is necessry to provide control of inclusions nucletion nd trnsformtion processes during secondry steelmking. Such control is very importnt for preprtion of clen steel free from detrimentl non-metllic inclusions efore the ldle is sent for csting. Besides, for luminum non-killed steel grdes (cord steel, ril steel, wheel nd tire steel, etc.) it is importnt tht certin chemicl composition of metl efore csting would ensure miniml mount of crystlliztion inclusions, their composition nd dispersion eing optiml. It is known tht microinclusions re removed from metl th much worse thn mcroinclusions. Therefore the minimiztion of microinclusions nucletion during secondry steelmking (especilly during deoxidtion) is importnt for clen steel production. It is known tht oxide inclusions size depends on ctivity of oxygen nd deoxidizing gent into liquid metl 7, 8. For exmple, deoxidtion of iron y luminum results in corse Al 2 O 3 inclusions (which re esily removed) if initil solule oxygen is 200 ppm or more 9 nd fine lumin inclusions (which re slowly removed) if initil oxygen is low (tle 1). The min method of steel refining from inclusions is stirring performed in two stges: t first vigorously to ensure cogultion, nd then wekly to llow the inclusions cogulted to e removed. Minimiztion of microinclusions nucletion in liquid steel is possile y mens of ppliction of optiml vrints of melt deoxidtion. In principle it is possile to use two contrry wys. The first stipultes entering of deoxidizers into melt with high oxygen ctivity (200 ppm nd more). The second is entering of deoxidizers into melt with low oxygen ctivity (less thn 40 ppm). Wys of preliminry oxygen ctivity reduction will e considered hereinfter. The first pproch is sed on genertion of corse esily removle inclusions; the second vrint is imed t inclusion mount minimiztion. For selection of n optiml vrint, it is necessry to tke into ccount ville secondry steelmking units nd melted steel grdes. The first method is relized in prctice during deoxidtion of LCAK- nd ULC-steels fter vcuum-oxygen decruriztion e.g. The oxygen ctivity in the melt is enough to generte corse lumin inclusions. Typicl inclusions extrcted from steel proe (which ws smpled in1 min. fter luminum ddition to 250 t melt in RH-degsser) re presented on fig They re corse dendrite lumin inclusions successfully removed within severl minutes of stirring. These inclusions re sent in proes smpled fter 15 min. of stirring (fig 5 ). There re some clusters from microinclusions (< 5 µm) present in this proe. In prctice, out 90 % of oxide inclusions re removed fter killing of LCAK- nd ULC-steels in RH-degssers (fig. 6) 6. Another deoxidtion procedure is relized during secondry steelmking of low- nd mediumcron steels with preliminry vcuum-cron nd slg-tretment deoxidtion 11, 12. It is known from prctice tht oxygen ctivity in steel cn e sfely decresed to 100 ppm nd less during vcuum-cron deoxidtion, if initil cron content is higher thn 0.05 wt. pct. Further oxygen ctivity reduction till ppm is possile y refining slg tretment. For tht purpose, slg is deoxidized y ddition of pproprite gents, e.g. SiC or CC 2. Owing to this preliminry steel deoxidtion the mount of inclusions generted fter luminum or titnium ddition decreses (nmely, quntity of inclusions of dimeter more thn 4 µm is less thn 15 sm ). The similr effect of preliminry high oxidtion level reduction my e reched y luminum ddition during tpping. But this method is less relile due to vritions of luminum losses during tpping from melt to melt.

3 3 Regrding the quntity of generted fine inclusions, the worst vrint is deoxidizer ddition into melt with men vlue of oxygen ctivity ( ppm) or its repeted trimming dditions. In this cse lrge quntity of microinclusions is generted. There re some other fctors tht cn influence liquid steel contmintion with microinclusions. One of them is input of Si-C cord wire. It is known tht clcium tretment improves cstility of Al-killed steels ut it my significntly increse steel contmintion with microinclusions, more thn threefold 13. Such impurity increse is indmissile for high-purity steel grdes. Therefore only miniml quntities of high qulity Si-C cord wire should e used. To reduce Si-C input without cstility degrdtion is possile y mens of ctive tretment of luminum killed steel y high CO slg under vcuum 11 or neutrl tmosphere. This technique llows to efficiently modify hrd lumin inclusions with minimum input of Si- C wire nd to ensure high steel purity. One more fctor, which increses contmintion of liquid steel, is mixing of two melts with different oxygen ctivity in tundish during ldle chnging. In this cse oxygen nd luminum from different melts re redundnt for ech other nd therefore they enter into rection. Another ctul prolem of smelting of steel with specified chrcteristics of nonmetllic inclusions is control of composition inclusions. It is ovious tht control of inclusions composition requires control of technologicl fctors which influence on inclusions ehvior. There re regimes of heting, slg forming, nd chnging chemicl composition of the melt. The lst fctor is significnt ecuse composition of microinclusions depends on chemicl composition of steel. Also, it is importnt to use qulity ferrolloys nd keep optimum regime of their dditions. Control of inclusions composition is especilly importnt in luminum non-killed high qulity steel grdes. The chemicl interction of oxide inclusions with liquid luminum non-killed steel hs one peculirity. It is considerle vrition of inclusions composition due to smll chnge of mounts of high oxygen ffinity elements or chnging of oxygen ctivity in the melt. For exmple Ti 2 O 3 frction in inclusions in low cron Ti-ering steel vries over wide rnge ccording to the mount of residul luminum (fig. 7) 1. The similr dependence is typicl for qulity high cron steels, where Al 2 O 3 frction drmticlly increses when luminum concentrtion exceeds 5 ppm. The influence of steel chemicl composition, temperture nd pressure ove the melt during secondry steelmking on composition nd mount of oxide inclusions hs complex chrcter. The computer thermodynmic simultion llows to tke into ccount the most significnt reltionships in the melt oxide inclusions system nd influence of min fctors on the occurring conversions. One of thermodynmic models developed for tht purpose is represented on we-site The internet version of the progrm Oxide inclusions is ville iid. This thermodynmic model hs the following ssumptions: 1) All elements re llocted uniformly throughout the melt ulk; 2) Inclusions re liquid nd sphericl; 3) Diffusion of elements in metl to or from the rection front is the limiting stge of inclusions growth or dissolution. The model does not include solid phse evolution in liquid oxide inclusions. But this fct is compensted y complex nlysis of computtion results nd pproprite phse digrms of oxide systems (such nlysis is utomted y developing of dtse on the sis of phse digrms). In this model it is ssumed tht the driving force of liquid inclusions trnsformtion is the difference etween concentrtions of components in metl volume nd metl inclusion oundry. Increse nd decrese of components in the inclusions re proportionl to corresponding flows: 4 ([ v J rd ] R] [ R] ), (1) R [ R

4 4 where J R is component R flow, r is inclusion rdius, D [R] is diffusion coefficient of component R in liquid steel, [R] v, [R] re concentrtions of component R in the ulk of metl nd on the oundry metl - inclusion. It is ssumed tht component concentrtion on the oundry [R] is in equilirium with oxide inclusion t ech itertion step. Ech component hs its own difference etween its concentrtion in metl volume [R] v nd t the metl inclusion oundry [R]. The [R] v vlue is defined y initil metl composition nd susequent itertive ltertions [R] v, which re computed tking into ccount mteril lnce of component redistriution. The [R] vlue is defended for ech itertion step from eqution: ] O ] were [ R, [ re ctivities of element R nd oxygen in the metl on the oundry of metl oxide inclusion, K ( R n O m ) is equilirium constnt for rection: (R n O m ) = n[r] + m[o], is oxide (R n O m ) ctivity in the oxide inclusion. ( R n O m ) Oxygen ctivity t the phse oundry is unmiguously defined for ech moment from condition of qusi-sttionry ehvior of mss trnsfer processes 15. It mens lnce of flows of oxygen nd other components from the ulk metl to inclusions oundry nd vice vers, when eqution (2) is true for ll the components. Being formlized, it my e written s the following eqution system: 1/ n1 K1 ( R ) 1 ] m1 / 1 [ R n [ O] K 1/ n 2 2 ] m2 / [ R n [ O] ( R.. (3) K 1/ n i i [ Ri ] mi / ni i [ O] 2 ( R ) ) ni J Ri mj O

5 5 At ech step, current discrete ccretions of msses of liquid oxide inclusion components re computed. Correspondingly, content of these components in metl chnges on the sis of conservtion of mteril lnce. The progrm computes system consisting of 11 elements, they re Fe, Mn, Cr, Si, Ti, Al, C, Mg, C, O, S. It lso clcultes nitrogen influence on components ctivity in metl, nd temperture dependence of rection equilirium constnts nd component ctivity in metl nd oxide inclusions. Component ctivity in oxide inclusions is clculted on the sis of A. Ponomrenko theory 16, nd in metl - on the sis of Wgner method. This progrm ws used for simultion of inclusions trnsformtion during secondry tretment of vrious steel grdes. The investigtions were executed t steel plnts of such compnies us CherMK, ZpSi nd Amurmetl. The received computtion results in comintion with SEM tests of the proes hve permitted to considerly increse efficiency of investigtions nd to improve secondry tretment technologies 11, 12, 17, nd 18. Appliction of the progrm in comintion with GIBBS melt control system 19 gives even more dvntges. In this cse the control of inclusions chrcteristics during secondry steelmking nd csting is rel tsk. GIBBS control system is sed on models of liquid metl tretment (which re developed for specific steel grdes) nd n ppliction progrm pckge to perform thermodynmic computtion, mteril lnce nd energy lnce computtion tking into ccount peculirities of the metllurgicl units. It should e noted tht interctions in the metl slg gs system re essentil to ll steelmking processes. It is considered tht every element i of the system exists in ll the three phses nd its mss is summed s: m i = m i + m ( i ) + m i. The distriution of the element etween phses is defined y the equlity of its chemicl potentil in ech phse, which ensures the system energy lnce: μ i = μ ( i ) = μ i. After input of chrged mterils mss, pplied pressure nd het vlues into the GIBBS progrm, equilirium stte of the system metl slg gs (mss, composition, nd temperture of metl, slg, nd wste gs) is computed. It is ovious tht most of rel steelmking processes re not equilirium ut it is very importnt to know the equilirium point of the system. It permits to clculte the system motion direction nd fctors determining chnges of its phses. Degree of equilirium point unttinility (lnce kinetic offset) in rel melt is defined y the only fctor. It is mixing rte. This fctor depends on low rte, th geometry, temperture, nd other fctors, which re individul for ech prticulr metllurgicl unit. The kinetic offset is defined during tuning melts. After djustment for prticulr steelmking unit, the GIBBS pckge precisely computes the whole complex of mss- nd het trnsfer processes. It permits to simulte the technologicl process course nd to predict chnges of ll sic prmeters of melt from its strt to the finish, such us melt temperture nd chemicl composition of melt, slg nd wste gs. The opertion of the secondry steelmking GIBBS control system t Belorusskiy metllurgicl works (RUP BMZ, Zhloin city) hs shown high ccurcy of prediction of steel chemicl composition nd temperture. It ensured troule-free work in the utomtic process control mode. Implementtion of the system hs llowed to stilize the process nd improve its monitoring 19.

6 6 Precise design of the trjectory of steel chemicl composition nd temperture chnge during secondry steelmking nd csting s well s the possiility of relile ppliction of the worked out technology in prctice thought the instrumentlity of GIBBS control system, gives wy to effective control of inclusions chrcteristics in such steel grdes s cord, ril, pipe steel, etc. In the sis of n on-line computtion of current inclusions composition nd mount in liquid steel there lies simultion of metl slg gs inclusions system chnging (fig. 8). It s executed for prticulr steel ldle during ll secondry tretment from tping up to csting. During this simultion, the control system lso computes the mount of inclusions removed from the melt. The ville knowledge se nd simultors of metllurgicl processes permit to considerly increse oth efficiency of steelmking process design nd reliility of the process implementtion y mens of nonmetllic inclusions control in the course of secondry tretment nd csting. References: 1. S. Ogiyshi Advnces in technology of oxide metllurgy Nippon steel technicl report. No pp L. Zhng nd B.G. Thoms Stte of the rt in evlution nd control of steel clenliness. ISIJ Int., Vol. 43 (2003), No 3, pp H. Tnk, R. Nishihr, I. Kitgw nd R. Tsujino Quntittive nlysis of contmintion of molten steel in tundish. ISIJ Int., Vol. 33 (1993), No 12, pp H. Tnk, R. Nishihr, R. Miur, et l. Technology of clening of molten steel in tundish. ISIJ Int., Vol. 34 (1994), No 11, pp P. Covch, K. Kijc, V. Msek, et l. Steel clenliness improvement through tundish configurtion optimizing. Metlurgij, 42 (2003) 4, pp L. Zhng nd B.G. Thoms, K. Ci, et l. Inclusions investigtion during clen steel production t Bosteel. ISS Tech 2003 (Conf. Proc.), Indinpolis, IN, USA, April.27-30, 2003, ISS-AIME, Wrrendle, PA, 2003, pp D. Y. Povolotskii Deoxidtion of steel. Мoscow, Metllurgiy p. [Rus]. 8. E. Steinmetz, H-U. Lindenerg, W. Mörsdorf nd P. Hmmerschmid/ Sthl u. Eisen., 97 (1977) 23, Secondry Steelmking Simultion User Mnul. The University of Liverpool Y. Miki, B.G. Thoms, A. Denissov, Y. Shimd Model of Inclusion Removl during RH Degssing of Steel. Iron nd Steelmker, Vol. 24, No , pp A.A. Alexeenko, E.V. Biekov, S.N. Kuznetsov, et l. Effect of some technologicl fctors on the cstility of n Al-killed steel in continuous illet cster. Russin Metllurgy (Metlly), Vol. 2007, No. 7, pp A.A. Alexeenko, S.N. Kuznetsov, A.G. Ponomrenko, et l. The influence of Ti lloying of 0.1 % C steel on index of corse oxide inclusions in peripherl prt of continuous illets. Electrometllurgiy. 11 (2007), pp [Rus]. 13. Scmrd S., Mccio G. Effect of clcium in Al-Si killed clen steel. Lucchini. C.R.S pp A.A. Alexeenko Aout progrm Non-metllic inclusions. We-site D.A. Frnk-Kmenetskii Mss nd het trnsfer t chemicl kinetics. Moscow. Nuk p. [Rus].

7 V.A. Grigoryn, L.N. Belynchikov, A.Y. Stomchin Theoreticl sis of electric steelmking processes. Moscow. Metllurgiy p. [Rus] 17. A.A. Alexeenko, E.V. Biekov, S.N. Kuznetsov, et l. Prolem of nozzle clogging during continuous illet csting of luminum killed low-cron low silicon steel. Russin Metllurgy (Metlly), Vol. 2007, No. 7, pp A.A. Alexeenko, V.P. Komshukov, Yu. A. Seleznev, et l. Mechnism of Sulfur Influence on Nozzle Clogging During Continuously Cst of LCAK Steel. Conf. Modern technology nd equipment for secondry metllurgy nd continuous csting of steel Moscow, My The Theses. Stl 5 (2006), p R.V. Sinykov, M.P. Gulyev, R.N. Mrtinov, et l. Commercil development of GIBBS control system of secondry steelmking (LF) nd degssing t Belorusskiy metllurgicl works. Metl i lityo Ukrini. 3-4 (2005) pp [Rus]. Tle 1 Effect of initil dissolved oxygen nd stirring on the time required to chieve very low * level of inclusions 9 Initil dissolved oxygen, ppm No stirring Time, min. Stirring * Meets purity requirements to pipe steels for gs nd oil lines. Fig 1. SEM-photo of cluster consisting of microinclusions of lumin nd MnO. Al 2 O 3,FeO. Al 2 O 3 solid solution

8 8 Fig 2. SEM-photo of slg inclusion SiO 2 Fig. 3. SEM-photo of mngnese silicte se inclusion of reoxidtion origin

9 9 Fig. 4. Form nd size of inclusions depending on oxygen nd lumin ctivity in the melt 8 Fig. 5. SEM-photo of lumin inclusions extrcted from smples tken in 1 nd 15 min. fter Alkilling t RH-degsser 10

10 10 Fig. 6. Totl oxygen (T.O.) removl rte, f ij, during RH deoxidtion period 6. f ij is the rtio of T.O. deleted y j- moment to T.O. in the smple i tken in 1 min. fter Al-killing. Fig. 7. Influence of luminum in low-cron Ti-ering steel ([Ti] %) on percentge of Ti 2 O 3 in oxide inclusions 1.

11 11 Strt Dt gining from previous step: 1. Melt: mss, composition. 2. Slg: mss, composition. 3. Inclusions: totl mss, composition. 4. Temperture. 5. Pressure. Dt gining out influences on the system during time τ: 1. Slg-forming mterils: mss, composition; 2. Deoxidizers, lloying nd modifying dditions: mss, composition; 3. Energy nd fuel inputted, het losses: quntity. Block 1. Computtion of inclusion trnsformtion fter ddition of deoxidizers, ferrolloys, nd modifiers nd evlution of prtil inclusions ssimiltion y slg nd slg emulsifiction into the metl. Block 2. Computtion of chemicl composition of metl slg gs system nd temperture tking into ccount dded slg-forming mterils, pplied energy nd fuel, nd het losses. Block 3. Computtion of inclusions trnsformtion which tkes plce due to chnging of metl composition nd temperture computed in Block 2. Sving of computed prmeters. End Fig. 8. Schemtic digrm of computtion of metl slg gs inclusions system current stte