The effect of rare earth metals (RE)  on  composition of carbon steel and the quantity, size and  morphologies of inclusions were experimentaly  determined. Optical scann-ing electron microscopy  and X-ray  diffraction analysis

as well as empirical  calculations  resulted that the RE

affected the chemical  composition  of  carbon steel and inclusions. The sulphur content did not vary. The inclu-sion quantity was increased. The RE modified inclusions, especially sulphides into  small-smooth size  under 4um. The steels containing from 0.01% to 0.02%RE have  develo-

ped globular oxysulphide inclusions of lb type, where as steels containing from 0.04% to 0.043% RE contain angular sulphide inclusions of Illb type. When the RE content in steel was over 0.062%RE harmful cluster  sulphide  inclu-sions of IV type was formed.Thus it  was  concluded  that 

the RE beneficial affected the formation and modification of non-metallic inclusions in carbon steel

Duy, C To

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C. To Duy Non-metallic inclusions in steelsNON-METALLIC INCLUSIONS IN CARBON STEEL CONTAININGRARE EARTH METALSTo Duy Ca * National Metallurgical Laboratory/CSIR, Jamshedpur 831 007*7he author is at present Post Doctoral Fellow at NMI .3CSIR- JamshedpurDr. Manager of Department , on le ,re .from Institute of Materials Science/VN('NSTVice president , General Secretary of Hanoi Association of Foundry and MetallurgyNghia do - Th liem - Ha not - VietnamABSTRACTThe effect of rare earth metals (RE) on composition of carbon steel and the quantity, sizeand morphologies of inclusions were experimentaly determined. Optical scanning electronmicroscopy and X-ray diffraction analysis as well as empirical calculations resulted that the REaffected the chemical composition of carbon steel and inclusions. The sulphur content did notvary. The inclusion quantity was increased. The RE modified inclusions, especially sulphides intosmall-smooth size under 4lim. The steels containing from 0.01% to 0.02%RE have developedglobular oxysulphide inclusions of lb type, where as steels containing from 0.04% to 0.043% REcontain angular sulphide inclusions of Illb type. When the RE content in steel was over0.062%RE harmful cluster sulphide inclusions of IV type was formed.Thus it was concluded that the RE beneficial affected the formation and modification ofnon-metallic inclusions in carbon steel.KEYWORDS: Rare earth metals, inclusion, carbon steels1. INTRODUCTIONThe carbon steels , specially cast steels, contain significant amount of inclusions as well asstringer and ribbon manganese sulphides and macro-cluster aluminates^'-'). They are detrimental tosteels, because they decrease mechanical, machinability and workability of steels1813j. A study onRE addition to carbon steel is necessary 114- " ). The RE are known to dissolve ribbon manganeseinclusions of MnS and macro-cluster aluminates to refine small globular sulphide inclusions of lbtype"'-22). These inclusions were distributed uniformly in inner crystal grains of steel when the REresidual contents and ratio RE/S were convenient 123'3p1 The heterogeneous angular sulphideinclusions of Illb type were formed when amount of RE residual and ratio Al+Si/RE wereoptimal. These inclusions are less harmful than type 11 inclusion, When amount of residual RE iswell over the threshold of cluster of globular sulphides of IV type. These inclusions were moreharmful than I type sulphide inclusions during machining and deforming (3140). The inclusions were1 1often smaller than 4µm and were undel'orrned`3'*4"t. The researchers were discussing more aboutthe influence of RE additions on the desulphurization ability but little about the RE/S ratioregarding inclusion formation.The present work carried out Iwo series of experiments on the effect of*RRF on chemicalcuurposillon of sleet and inclusion, on itR'Iusiorr's tluiuitity, size and nrorphology2. EXPERIMENTALTwo sery of experiments with 8 samples each were carried out in the laboratory The REadditions into low-carbon and high-carbon steels were varied from 0.5 to 2.0 kg/t. The REadditions were in the form of FeSiRE which composition as following: 25%RE (IO%Ce and 7-8%La), 25%Si, 3%Ca, 5%A1 and Fe remaining. The starting steels compositions are shown intable I.Table 1. Chemical compositions of RE free samplesstee s chemica l content ( wt. /o)C Mn Si P S_ Al, 0, N2low-carbon 0.29 0.33 012 0.0210,015 0.12 0.00726 0.0094high-carbon 0.58 0 .18 0.11 0.015 0.024 0.07 0.0205 0.0062Experiments were carried out in an induction furnace of type GV-22 with a maximumelectric power of 22kW. The carbon steels were melted in A1203 crucibles (99%A1203. in the formof ingot. The melting process was as following: the steels were heated to a temperature of1600°C to 1620°C measured with a PtRhPt 16 thermocouple, then it was deoxidized by aluminiumwire. After one minute, the sample was taken and symbolized "o" (it means free RE). The REswere added into melts with varying amounts of 0.5, 1.0, 2.0 kg/t for each steel.All samples were cast into copper moulds. These samples were treated and machined foranalysis and measurement. The chemical compositions of steels were analyzed quantitatively. Theoxygen, nitrogen and oxide phases were analyzed in LECO-TC436 machine. The microstructureanalysis was carried out rnicroptical microscope (Neophot II). Selected samples f rorn each seriewere examined with a SEM ( JEOL-35C'F and FDX by LINK 860/11), The type of inclusionswere determined in the X-ray diffractometer (DX5000).3. RESULTS AND DISCUSSIONS3.1. Cleanliness of steelsThe results of chemical composition of steels were shown in table 2.The table 2 shows that residual content of RE in steel ranges from 0.007%to 0.094%(from 0.005% to 0.061%Ce). This result corresponds with point A of figure 14U,2f5, ppmFigure 1. Schematic of reaction paths for sequential and coupled deoxidation-desulphurization reactions'""Table 2 . Chemical composition of steel samplessample (w_ t.%)No C Mn Si P S Al, 02 Ce LaA,A2A3D,D2D30.032 0.05 0.04 0.015 0.021 0.019 0.00806 0.005 0.0020.120 0.30 0.32 0.015 0.021 0.010 0.00843 0.021 0.0200.030 0.16 0.43 0.016 0.020 0.070 0.01920 0.0280.0350.590 0.28 0.17 0.015 0.024 0.070 0.01310 0.012 0.0080.660 0.46 0.37 0.016 0.024 0.075 0.01260 0.025 0.0140.620 0.41 0.64 0.016 0.024 0.078 0.00980 0.061 0.033RE N20.007 0.006200.041 0.014750.063 0.006490.020 0.007040.039 0,011900.094 0.01110The table 2 also shows that when RE addition was increased from 0.05 kg/t to 2.0 kg/t thelow-carbon steels has varied sulphur content, but high-carbon steels show no change in sulphurcontent . These can be explained from modification side of sulphide inclusions i.e., meanssulphide shape control. In the sample A3 of sulphur content was reduced with RE addition to 2kg/t. The RE addition also decreased oxygen contents in high-carbon steels and in contrast theoxygen content in low-carbon steels were increased The RE dissolves in high-carbon steels morethan in low-carbon steels as shows table 2.The reciprocal effects of RE, 0, S depend on ratios of RE/S which can be seen in figure 2.The figure 2 shows that the RE additions into high-carbon steel have reduced oxygencontent . Where as low-carbon steels increased oxygen contents. In fact, the RE additions affectedthe deoxidation of steels. The oxygen from atmosphere and around steel tapping current reactedwith RE in steel from point E with back direction to point F. The reactions occurred as follows:2/RE/ + 2{0} ,,,,, + /S/ - /RE202S/ (1)and then : /RE202S/ + 2/S/ -* /RE2S3/ + /0/ (2)3001500120 ONO OMI10 00300f 0003d-0 OOB-0 009-0012-0015T - 1-r-b-an al---_RU ratio5 35Figure 2 . The relationship between da(On,_r Rr-Oancr u) and RE/S ratio in steelsThis oxygen was transferred into steels, resulting an increase of oxygen contents in low-carbonsteel . The reoxidation of steel was important problem j42j.3.2 The chemical composition of inclusionsThe results of chemical compositions of inclusions were shown in table 3.Table 3. Chemical composition of inclusions in steelssample number area number area area/number chemical content (wt.%)No - µm2 cm-2 % µm2 Al Si Ca S Mn REAoA,A2A3DotD,D2D3271 1798 10170 0.0675 6.6 15 2 2 43 37 0244 5184 10988 0.2334 21.2 42 38 3 0 12 5258 1743 3417 0.0231 6.8 13 1 2 27 5 52261 1483 8398 0.0477 5.7 9 0 3 13 1 74273 2463 6830 0.0616 9,0 20 17 3 47 14 0255 1941 8203 0.0624 7.6 17 14 3 36 20 10262 1495 7374 00421 5.7 9 2 2 30 21 36255 2342 19139 01758 9.2 3 0 3 17 1 76The table 3 shows that before additions of RE, aluminates and manganese sulphides werepresent in the steel. The addition of RE into steel reduced the sulphide quantity, especially insample A, any sulphide wasn't observed. In fact, manganese sulphide also were not appeared.This can be explained through following reactions:When RE content reached to 0.008% ( AI+Si/RE > 1) REA]I,Olg forms (15,4347)/RE/ + 6/A12O3/ = /REA111O18/ (3)When RE content was about 0.02%, REAIO3 forms (43-47)^/RE/ + /AI/ + 3/0/ _ /REAIO3/ (4)and /RE/ + /AI2O3/;,1C,. _ /REAIO3/ + /Al/ (5)The quantitative examinations by SEM, EDX and X-ray resulted that the residual REcontent from 0.02% to 0.07% formed oxysulphide cores (RE2O2S).The peels were surrounded bycores with stechiometric sulphides (REhSY). There was another case where REAIO2S formed.4 4IThese results accorded with previow; works'"' 17'''"'4'' S°). When RE contents in steel in high (over0.07%) RES"5'4'') appeared and simple RE oxide was not observed. This result agrees withrecently published works''"•44,50)When the RE addition is raised to 2.0 kg/t MnS gets dissolved as following:2/RE/ + 2iO/ 4- /MnS/ = /RE202S/ + /Mn/ (6)Thus RE reacted to form mainly oxysulphides and heterogeneous oxysulphides alongpaths BC, B'C' and FG in figure 1 "l he RE afl'ected the removal of cluster aluminates and ribbonmanganese sulphides according to reactions (4), (5) and (6).3.3. Quantity and size of inclusions in steel containing REInclusion quantity and size distribution are shown in figure 3, where the relationship of REaddition to the inclusion number and size in two low-carbon and high-carbon steels were shown insample A2, there was the lowest number of total inclusion when RE was added in the ture of1 kg/t . There was total of 3417 inclusions per cm2(shows figure3). While total oxide inclusionsare very low, e. g., 676 inclusions per cm2 (shows figure 4).8000700060005000400030002000100001200010000800060004000f^7low-carbon steel2 34 56 78910 23 45678910 234 5878910 2 3456 78910>10Inclusion size lµnVhigh-carbon steel2000 Jil-0 IIMMM M - ._ »..--. I1 JI11l_l.. L2 34 5678910 2 3 4 5 6 7 8910 2 3 456 78910 23 45 87 8910>10Inclusion size /y mlFigure 3 . Effect of RE addition on inclusion number and inclusion size in carbon steel5 5Inclusion size, I ym/Figure 4. Oxide and sulphide numbers in low-carbon steel containing RE( series A )The total inclusion number «as the lowest in sample D2 when RE added in the ture of1.0 kg/t (figure 3). The total oxide number was obtained to be 1041 inclusions per cm2 (showsfigures).16000i140001200011 100008000CC.2 6000400020000RE free RE of 0.5kgrt RE Of 1.0k9kox de sul hideRE or 2OkgR1H K__3. A __J_L_M_ _400 A' fl-m -111111111110 ^M A -am4 10 >10 4 10 >10 4 10 >10 4 10 >10Inclusion size Iii m/Figure 5. Oxide and sulphide numbers in high-carbon steel containing RE ( series D )The general observation in most samples containing RE is the numbers of inclusion haveincreased compared with the RE free samples. This problem can be explained as follows: theadded RE into steel reacted with oxides and sulphides presenting in steel and refined them intomore small &smooth inclusions. These inclusions distributed uniformed in the sample. Theseinclusions were refined to size under 4 pm, so that they can not be harmful to steel duringdeformation and machining. In fact, there were about 86 to 96% inclusion refinement to 1-4 µmsize range and not inclusion over I ultnt. The samples of serie D shows that the added RE to 1.0kg/t can remain in this stec, where as 10.042° oRE results the smallest inclusion sizes.The RE affects surface tension of inclusions, especially surpresses and restricts surfaceactivity of MnS. The RE oxides and sulphides are thermodynamically stabile, The value ofsurface tension of RE inclusion was ecry high, and therefore have not grown in size. It is animportant character of inclusion modification.3.4. Morphology of inclusions in steel containing REThe results about studies of inclusion morphologies were shown in figures 6, 7, 8 and 9,The figures shows that RE lice steels only presented cluster aluminates , silicates andmanganese sulphides (shows figure 6.)nA Al Si Ca S Mn La Ce O (wt%)10.8 23.7 0.6 0.9 13.8 0.3 0 79.9Figure 6 . Optical micrograph of an inclusion in low-carbon steel before RE addingin sample A„ , x500The low-carbon steel containing RE ( 0.5 kg/t) were observed to have globular sulphides of lbtype. The complex oxysulphides containing Al, Si, RE also appeared (shows figure 7.). Thus theresidual RE contents in steel from 0.01% to 0.02% formed globular oxysulphides of lb type. This7 7Al ('a S Mn Mg La Ce 0 (wt%)A 8 I 21 1 I 22 31 15B 19 - 12 I 1 18 26 23Figure 7. The typical inclusion of Ib type in low-carbon steel after RE adding to 0.5 kg/tin sample A,_ x500,result was accorded with recent work'"). When RE addition raised to 1.0 kg/t andremained in steel from 0.041% to 0.0440%%RE, angular heterogeneous sulphides andoxysulphides of 11Ib type formed. These inclusions contained RE, Al and Mn (showsfigure 8.). These inclusions were less harmful during machining than inclusions of I type.When the RE additions were raised to 2.0 kg/t the remaining RE contents in steel change from0.062%RE to 0.094%, causing the formation of typical cluster oxysulphides of IV type (showsfigure 9). These kinds of inclusion are more harmful than inclusions of I type. From X-ray analysisand composition the globular inclusions of lb type were formed to have the stoichiometrs RE2O2Stype. The angular inclusion of Illb type have stochiornetrs are of REAIO2S . The clusterheterogeneous inclusions of IV type are of the following type : the cores were RE2O2S and thepeels were RES or RE,S,.. These results are not in contrast with our previous worksr19, 24,25 .33.34)8 8.3Al ('a Si S Mn La Ce 0 (wt%)A 51,9 0.1 0 0.1 0 1.6 3.6 41.713 161 02 0.8 0 0 12.5 27.8 42.6Figure 8 . The typical inclusion of Illb type in low-carbon steel after 1.0 kg/t RE additionin sample A,, x500^ f •Figure 9. The typical inclusion of IV type in high - carbon steel after 2 . 0 kg/t RE additionin sample U:. x500From the results cited above excessive RE addition is not needed , otherwise it may causeharmful effects like reduction of mechanical properties and machinability of steel andcleanliness.In the present experimental conditions, it can be emphasized that the RE additions did notreduce sulphur contents in steels . I he RE sulphides formed are of high densities , and thereforecould not be floated to slag and thev remained in steel . The affects of RE additions wereconduced from modolication aspect It was globularization , refinement and uniform distributionof inclusions The RE additions removed macrocluster aluminates, and ribbon manganeseinclusions . These characters of RI'. inclusions are essential.9921. V.I. Javojskiy a kol. Nemetalicheskiye vklucheniya I svojstva stall, M. Metallurgiya (1980), 17622. O. Volny and S. Mrovec, Vyzkunina zprava VUIIZ, Dobra-Czech (1988),l23. ). Volny, Vyzkumna zprava VV I 17., Dobra-Czech (1985),5, 2524.C. To Duy, L. Dobrovsky and 1'. Kozelsky, I. Science and l echnolo, 32, (1994), 5, 4525. C. To Duy, ,/..Science and Technology 34, (1996), 2, 4226. Z. Buzek, Sbornik konference V I:O I Ostrava (1973), 7727. S. Liu et al. Actcr metcrlurg Sinn'u (1984), 20, 5p-A32228. H. Weiss, Stahl and 1•,isen (1974), 10, 47729. J.I. Matrasov a kol.Aletallovedc'nit•e i tcrmicheskaya obrabotkaMetalov( I978),6,56(ncxtMIT'OM)30. K.N. Gurasev a kol. Lileynoye proizvodsiro (1970), 3,3331. N.F. Sidorenko, Liteynove proi-rochtvo (1970), 3,2832. A.N. Gladich a kol. A417 OM, (I ')70), 3, 3133. C. To Duy, Eflect of REM on sulphide inclusions in steel rolling pipe, I lanoi 1997,3(unpublished report of ISM)34. C. To Duy, A study on the desulphirization and sulphide inclusion in I2('r20Ni38Mo5 alloy byCaO-MgO-R1`-Cale synthetic powder, I lanoi 1997,5 ( unpublished report of ISM)35. C. To Duy, ./. ('hcmi.ctrt' 34, (1 000), 6, 32 (in Vietnamese)36. Y. Nui et al, 7i-ans. ISI,I 22, (l 089), 40837. Y. Nui et al, Trans. IS/I22, (1982), 39938. B.L. Bramfitt, Ale!. Trans. 1, (1970), 198439. A.J. Akselrod a kol. Izv. VI/Z. (7wrnaya Metallurgiya 30, (1986), 12, 5940. C To Duy et al. Annual report of IFM, Thai Nguyen, (1982),.414I . W. K. Lu and A Mclean IronmaI ing and ,Steeluiking (/974), 1, (4), 22842. 0 Volny, Kandidatska disertacni pi ace VSE3 Ostrava, (1981)43. S. Malin, ,Sewn. .1. of Afetall:ugt , ( 1976), 5, 24844. W.O. Philbrok et al. Inter. Metals Reviews 220, (1977), 18745. W. G. Wilson et al. Metals progress 104, (1973), 7, 7546. W.G. Wilson et al. J. of NfetalR, (1985), 9, 3647. A.G. Tiurin a kol. Irv. AN SSSR .Afetcrllt•, (1982), 6, 4848. W.G. Wilson et al. J. ofAletalR, ( 1974), 5, 1449. Y. Dong and Q I Ian, Steel Rewarch 68, (1997), 9, 38350, W. Henderson et al Pioc. of ('on' AI ME " Electric Furnace" 31, (1973), 162

Non-Metallic Inclusions in Carbon Steel Containing Rare Earth Metals

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Non-Metallic Inclusions in Carbon Steel Containing Rare Earth Metals

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