ISIJ International. Vol. 35 (1995), No, 3, pp. 286-291

Effect of OxYgenContent on Size Distribution of Oxides in Steel

Hiroki GOTO.Ken-ichi MIYAZAWAand Kazuaki TANAKA1) Kimitsu R&DLaboratory. Nippon Steel Corporation, Kimitsu. Kimitsu. Chiba-ken, 299-1 1Japan. 1) Kimitsu Works. Nippon Steel Corporation. Kimitsu, Kimitsu, Chiba-ken, 299-1 1Japan. (Received on September22. 1994, accepted in final form on November18. 1994)

The effect of the oxygen content in steel on the size distribution of oxides was investigated using Ti deoxidized steels. The numberand size of oxides in the continuously cast steel slabs have been observed and the volume fraction of the fine oxides and the amountof oxygen precipitated as oxides during cooling and solidification have been examined. The results obtained are as follows. The numberand diameter of the fine complex oxides composedof mainly Ti.O.. AI.O, and MnOincrease with increasing the oxygen content in the steel. Almost all the oxygen in the steel is existing as oxides, namely the oxygen content of the steel is almost equal to the amountof the oxides. The oxides precipitated during the cooling and solidification correspond to about 70"/* of the total oxide amountin the steel and are mostly smaller than I O,lm in size. Most of oxides present in molten steel before casting are also fine because these fine oxides can not float and separate from the molten steel and are resultantly suspendedin the molten steel. The oxides smaller than around I0,Im occupy a large proportion of the total oxide content in the steel and govern the oxygen content in the steel. KEYWORDS:oxygen content oxide; deoxidation; titanium; precipitation,

1. Introduction 2. Experimental Procedure Becauseoxides larger than roughly 50 defects Thelow carbon steels (C: 0.06-0. Si: O.05~).26, Mn: pmcause II. in steel products,1) manystudies have been done so far l.20-1.54, Ti: 0.006-0.016, O: 0.0017~).0039masso/o) predominantly on the relatively large oxides. However, were used as experimental materials. These steels were oxides formed during solidification of steel are relative- produced by the processes of BOFsteelmaking, second- ly small andhavebeenrarely investigated. Recently, tech- ary refining and continuous casting. Specimens were nology has been developed for controlling the mechani- cut from continuously cast slabs in the thickness di- cal properties of steel by making use of oxides smaller rection and oxide distribution in the specimens was than a few micrometers in size, which are formed during observed. Numberand size of oxides and the morphol- solidification, as nuclei for transformation of steel and ogy were observed by optical microscopy with I OOO precipitation of sulphide.2) Nowthe importance of the magnification. Thenumberof oxides wascounted in the researches on fine oxides is increasing. On the oxides view area of 1250mm2for the oxides larger than 10 ,tm formed during the solidification of steel, it is necessary and 4mm2for the oxides smaller than 10,lm. It was to investigate the behavior an~ size distribution in a size difficult to identify oxides less than O.5 klm by the optical range from submicron to a few 10 ~m. Since precipitation microscope with I OOOmagnification. Theminimumsize behavior of the oxides during the solidification and size of oxides that can be observed in this study is estimated distribution would be affected by oxygen content in the to be 0.5~m. The composition of oxides was analyzed steel, it is important to elucidate effect of the oxygen by an electron-probe microanalyzer (EPMA). In order content on the numberand size distribution. to discuss the effect of oxygen content on the oxide Although effects of cooling rate on the precipitation distribution by keeping the cooling and solidification behavior and composition of the fine oxides during rates and solidification structure constant respectively, solidification have been reported by the authors,3'4) Iess the distribution of oxides at the position of 60mmfrom information is available on the relationship between the lower slab surface in the thickness direction (position oxygen content in steels and precipitation of the fine of a quarter thickness) wasinvestigated. oxides. In this study, in order to understand the fundamental phenomenaof the fine oxides formation 3. Experimental Results during the cooling and solidification of molten steel, the 3.1. Size Distribution and Composition of Oxides effect of oxygencontent on the size distribution of oxides in Ti deoxidized steel has been investigated. The size distribution of oxides in the specimens of different oxygen contents is shown in Fig. l. Oxygen

C 1995 ISIJ 286 ISIJ International, Vol. 35 (1995), No. 3 content of molten steel in tundish was measured. The size of oxides are shownin Figs. 2and 3, respectively. numberof oxides increases with decreasing the diameter Figure 2showsthat the numberof oxides increases with of oxides and with increasing the oxygencontent. There increasing oxygen content. In Fig. 3, the average size of are manyoxides smaller than 10pmin diameter. The oxides slightly increases with increasing oxygencontent. effects of the oxygencontent on the numberand average The morphology of typical oxides is shownin Fig. 4. The composition of oxides analyzed by the EPMAare shownin Fig. andTable 1. Theoxide is nearly spherical c(r 102 5 and mainly contains Ti, Al, E o(ppm) Mnand O. The oxides E o 17 observed in this study are assumedto be complexoxides 101 27 of mainly ,\ C composed Ti203 and containing Al203 and 39 Mn0.5) E ',b :s 100 c *\ 3.2. Effect of OxygenContent on OxideAmountin Steel ~co \ In order estimate the in steel, 1:;o to amountof oxides the '~ O~1 \ I \ ~o ~:L5 * O~2 \ \ ~i \ *, co :sE O~3 ~4 z I > O IO 20 30 40 o 610 9 Diameter of oxide (vm) IS3 ~)OO Fig. 1. Size distribution of oxides in Ti deoxidized steels. o* e ~(DE2 c\lEE~ clj 30 :5 ~* 1 o)o ~ a5 E (s)P:OOO O o o* ~c 20 O IO 20 30 40 50 co Oxygencontent (ppm) a) O oO D Flg' 3' Relationship between oxygen content and average > O diameter of oxides in the steels. o 10 -o o* ~)E = Zo 1O 20 30 40 50 Oxygencontent (ppm) H-H10/Im

Fig. 2. Relationship between oxygen content and numberof Fig. 4. Morphology of oxide in Ti deoxidized steel by optical oxides in the steels. microscope.

rrl~~H~=,: :'(E]ass%) ~ i f i I : 60 i i li

LLLL !T l 5l/m 1' (massx)i i i l t f- 20. -} 20- l l : l i,: l o: 40 : : i I i (E]assX) 5~ I 6.0 ;__ l lLLi __ l ~30T:' lrl ! 40 ~ lo :l ~O._ o~ ! :i ~i 20 _ :. - ~5 -lO i Mn j = Fig. 5. ~-d i Chemical compositions of an oxide in Ti deoxidized o d :~ ~ ;i: I ~ steel. o *1, : : : ; l

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Table l. Chemical compositions of oxide in Ti deoxidized ~~OO steel. (masso/o) ~o > 0.02 o Ti203 Al203 MnO CaO Si02 MgO ~ O co O 78.2 12.9 4.7 0.8 1.1 0.9 12o > OO o O9oO H- the volumefraction of oxides wascalculated. Thenumber O co 0.01 o of oxides per unit volume is calculated by DeHoff's :$:5 equations,6) expressed by Eqs. (1) and (2), and by using o O the numberand size of oxdes observed by the optical a5 H-* microscope. Thevolume fraction of oxides is calculated o by Eq. (3). E = 20 30 40 50 2N. """"""(1)6) ~ O IO N "" > I~ d ObServedoxygen content (ppm) Fig. 6. Relationship between observed oxygen content and 1 1 1 ¥¥¥¥¥¥¥¥¥¥¥¥(2)6) fraction in steels. ¥¥¥¥¥¥ volume of oxides the -=~-~ n di

E~!~ ~ 3 . (3) V= d ...... 6- N 50 where, Nv: number of oxides per unit volume in ~ ~c: specimen (m~3) o 40 N* : numberof oxides per unit area in specimen ~c 2) O (m~ o 30 O 8 di : apparent particle size of ith oxide among Oc g8 n oxides (m) o) >> 20 d: harmonic meanof oxide particle size (m) V: volume fraction of oxides. o lO O To investigate the relationship between the oxygen ~3 10 content and the of existing oxides in ~! amount oxygen as :: the steel, the content of oxygen existing as oxides in the ~2 steel wascalculated by using Eq. (4). O 10 20 30 40 50 ~Observed content (ppm) [O].* (p.*/pF*) V¥ (O)o* (4) oxygen = "' ""-"" Fig. 7. Relationship between observed oxygen content and where, [O]0> of existing oxides in : content oxygen as calculated oxygen content existing as oxides. steel p.* : oxide density ~~oo PF. steel density O~1 : ~o 1 (O).* : oxygen content of oxides. > The value of p.* is obtained from the weight meanof ~co 0~2 oxides by assuming that the oxide comp,osition is the o 1 ternary system of Ti203-Al203-MnO. The densities of D> the oxide follows; g/cm37)p(AI o 0~3 are as p(Ti 203) =4.6 203)= ~- 1 3.97 g/cm37) 5.45 g/cm37) and 7.86 g/cm3 8) o p(M.o) = pF. = c: The value of (O)o* is that measuredby the and EPMA :~5o O~4 shownin Fig. 5. 1 oCEl The relationship between the and the oxygen content H-L 0~5 volume fraction of oxides that indicates the of 1 amount E:o oxides in the steels is shownin Fig. 6. Thevolumefraction = of oxides increases with increasing the ~o 10~6 oxygen content. 10 20 30 40 The calculated content of oxygen existing as oxides > O Oxide diameter( hl ([O].*) and the observed oxygen content are compared m) in Fig. 7. The two values almost agree with each other. Fig. 8. Relationship between oxide diameter and volume This meansthat mostof the oxygenin the steel is present fraction of oxides. as oxides. The relationship between the size of oxides and the a maximumat the diameter of a few //m and is rapidly oxygen content were investigated. The volume fraction decreasing with increasing the oxide diameter. The of oxides for each range of oxide diameter is shownin volume fraction of the oxide larger than 10pmis ex- Fig. 8. It is seen in Fig. 8that the volume fraction has tremely small.

C 1995 ISIJ 288 ISIJ International, Vol. 35 (1 995). No. 3 E 80 ~ 50 70 - - O-' Total oxygencontent Q e- -- Soluble oxygen content observed content E - - -:: CLQ 60 Equilibrium value o 40 ~ ~c: o ~ 50 o oc \ o ~c: 40 \ c:o 30 O 8 30 ~ :\ o) C \\.~~:.~ c:o o o > 2:nu 20 o) ~'-O\ R *-o_ o O > 10 > C 1525'C c~s 10 O ~ o Equilibrium value in solid at 525~) IS 1 E:s 0.01 O O 0.02 0.03 O O IO 20 30 40 Titanium content (maSSo/o) Oxide diameter( Fig. ro. Reiationship titanium v m) between andoxygencontents. Fig' 9. Change in cumulative oxygen content with oxide diameter. contents, the activity coefficients of titanium and oxygen assumedto be andthe activity coefficient of were I, Ti203 The cumulative content of the existing the assumedto be In experiment, the oxygen as was I . our temperature oxides smaller than a diameter ([O]ex,')' which is cal- of molten steel is 1545 to 1580'C and the liquidus culated by Eq. (5), is shownin Fig. 9. temperature of the steels estimated by Hirai et al.lo) is l 520 to 1528'C. Theequilibrium value of titanium and [O]~*,.=(p~*/pF*) Vi ¥ (O)~* (5) ~ ..... ¥¥¥¥¥¥¥¥¥¥ oxygen at the average liquidus temperature of 1525'C where~Vi is summationof the volumefraction of oxides wascalculated by Eq. (7). smaller than an oxide diameter. The proportion of the The titanium and oxygen contents of solid steel were estimated follows. oxides smaller than an oxide diameter in the oxygen as Namely, assumingthe equilibrium solidification content of steel canbe knownfrom the cumulative oxygen where the equilibrium partition of solutes content. Namely, in Fig. 9, the calculated cumulative is maintained at the solid/liquid interface and the solute oxygen content increases till the oxide diameter of diffusion is complete both in the solid and liquid phases, 5-10 pmand becomesa constant value which is roughly the titanium and oxygen contents in solid steel during solidification equal to the observed oxygen content. This meansthat can be expressed by Eqs. (8) and (9), the amountof the fine oxides smaller than about lO,lm respectively. the content in steel. occupys oxygen [masso/oTi]*=kTi[masso/oTi] ...... ,...... (8)

[massoloO]*= ko[massoloO] ...... (9) 4. Discussion where, [masso/oTi]*: titanium content in the solid It has been found that the oxides smaller than l0/Im phase are strongly related to the content of steel. The oxygen [massoloO]*: oxygencontent in the solid phase reason is discussed in the followings. kTi : equilibrium partition coefficient 4.1. Precipitation and Growth of Oxide during Cooling of titanium and Solidification of Molten Steel ko : equilibrium partition coefficient of The oxides in molten steel can precipitate and grow oxygen. Eqs. (7) (9), the during two periods; during cooling of molten steel from From to equilibrium contents in the solid solidification teeming to liquidus temperature and during solidifica- at 1525'C on are given by Eq, (10). tion from liquidus to solidus rela- temperatures. The ([mass ~TI]./kT,) ([mass O]./ko)3=7.96 x l0~14 tionship ~ between the titanium and total oxygen con- .(9) tents in steel slab is shown in Fig. 10 together with where, kTi =0. 14 and ko 0.02 11) soluble oxygen content of molten steel. The oxygen = relationship betweenthe titanium the content increases with decreasing titanium content. The and oxygen contents is shownin Fig. 10. In this figure, the The equilibrium between titanium and oxygen at the amount of Ti203 precipitated during the cooling of molten steel liquidus temperature was obtained as follows. Since it and the solidification be estimated. Namely, has been confirmed that the oxides exist as Ti203 in can when the steel solidified steel,5) the deoxidation equilibrium of molten molten with an average titanium content of 0.012masso/o is cooled from 560'C, is steel can be calculated by using Eqs. (6) and (7). 1 which average temperature of molten steel, to 1525'C, the solubilities """"""¥¥¥¥¥¥¥¥¥¥¥¥¥¥(6) 21!:+3Q=Ti203 of titanium and oxygenchangeas indicated by the arrow the molten steel is solidified, the solubilities log(a~i ¥ag/aTi,03) 56060/T+18.08 (7)9) . When = ..... @ ~ of titanium and oxygen change as indicated by arrow Since the molten steel low in titanium and (~) The proportions of the oxides in the was oxygen ¥ amount of

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molten steel, the of oxides precipitated amounts during E*._1,~, cooling to the liquidus temperature and during OO solidification can be calculated from the proportions of d: 90 Diameter of oxide the vertical length of the arrows and @, O, @ ';~~ 80 respectively. Namely, since the soluble in the solid ~_oo oxygen d=30~m steel at 1525'C is almost zero, the soluble oxygen in the 70 d=20~m molten steel at 1560'C precipitates as oxides. The o 60 difference betweenthe observed total and soluble oxygen co 50 contents which is the vertical length of the in cr; arrow cl) 40 Fig. O ~ 10 cortesponds to the amountof the oxides existing 30 in the molten steel. The amountof oxides in the molten ~o) d=1 ~1 steel is 30o/o total 20 O m about of the amountof oxides, the :j== (TSO of oxides precipitated during cooling to the 10 amount d=5 ~1 m liquidus about o/o, the ~LL temperature 27 and amount of 50 oo oxides precipitated during solidification about 43 o/o This ~ 1 . Time min meansthat the oxides precipitated during cooling and ( ) Fig, 11, in solidification occupylarge percentage of the total amount Change fioating distance of fine oxides in molten steel with time. of the oxide in the steel slab.

4.2. Size of Primary Deoxidation Products Suspended ~~:oo in Molten Steel 0~1 IS 1 Theoxygen content in the molten steel is higher than Chemical compositions of the steel > (mass'/.) the in equilibrium co C:O.003 oxygen content with titanium, as ~a) O~2 Si:0.01 shownin Fig. lO. This be caused by the suspension 1:, 1 Mn:O,12 may '~ Al:0.026 of fine oxides in the molten steel. Theseparation behavior O 0~3 O:0,0023 of the suspending oxides smaller than 10,lm from the H~O 1 molten steel is discussed here. Theoxides in this case are = the deoxidation the O 0~4 primary and reoxidation products. ~= 1 Assuming that the oxides float in the molten steel oclS according to Stokes' Iaw expressed by Eq. (10), the ~= O~5 floating distance of oxides is given by Eq. (1 1). o 1 E:s v g(pF. P.*)d /1 8n (10) ~O 10~6 = - ...... O 10 20 30 40 L=v't ...... (11) > ..,,.. Oxide diameter( ~l m) where, v : floating velocity of oxides in molten steel d: diameter of oxides Fig. 12. Relationship between oxide diameter and volume fraction of oxides in Al deoxidized steel. n: viscosity of molten steel L: floating distance distribution of oxides in the steel the fraction t: time. volume wasobtained. Theresult is shownin Fig. 12. The oxides Thefloating distance for Ti203 of 5, lO, 20 and 30,tm smaller than ,Im large proportion of the total was estimated. The results are shownin Fig. 11. The IO occupy amountof oxide in the steel and are considered to physical properties used for the calculation are as fol- govern the in the steel like the 10ws; p..=4.60g/cm37), pF.=7.0g/cm3 12), n=0.048 oxygen content case of Ti de- poisel2) and 980cm/s2. oxidized steel. g= It confirmed that the oxides governing the FromFig. I l, it is evident that the floating distance was oxygen content in solidified steel are those smaller than lOpm decreases with decreasing oxide size. The distance over both for the Ti deoxidized steel in which large which the oxide of 10,lm can float for example during amount of the oxides precipitated during the cooling 120min in the static molten steel in ladle is estimated and solidification and for the Al deoxidized steel in which to be about 20cm. This value is small as comparedwith extremely less of the oxides precipitated during the bath height of 350 in the ladle. The percentage amount cm the period, of the oxides smaller than lO,lm to be floated and same separated is presumedto be very small. The result that 5. Conclusions the oxygencontent in the molten steel is higher than the soluble is oxygen content probably caused by that the The effect of the oxygen content in the steel on the oxides smaller than lOpmare suspendedin the molten size distribution of oxides was investigated using Ti steel. deoxidized steels. The results obtained are as follows; In this study, Al deoxidized steel in which primary (1) In the steel, the fine and spherical complexoxides deoxidation products are main oxides wasproduced by composedof mainly Ti203, Al203 and MnOhave been the sameprocesses as the case of Ti deoxidized steels to observed. With increasing the oxygencontent in the steel, comparethe behavior of oxide distribution betweenAl the numberof oxides increases and the diameter of the and Ti deoxidized steels. On the basis of the size oxides slightly increases.

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(2) The estimated content of oxygen existing as ox- REFERENCES ides in steel the the approximately agrees with observed 1) K. Misonoo, A. Fujii, R. Fukumoto and A, Nakajima: oxygencontent in the steel. Almost all the oxygen in the Tetsu-to-Hagan~, 68 (1982), 147. steel is present as oxides, namely the oxygen content in 2) J. Takamuraand S. Mizoguchi: Proc. 6th IISC. Vol. I ISIJ, (1990), the steel is almost equal to the amountof the oxides. Tokyo, 591. 3) H. Goto, K. Miyazawa, K. Yamaguchi, S. Ogibayashi and K. (3) oxides precipitated during the cooling and The Tanaka: ISIJ Int., 34 (1994), 414. solidification o/, the total correspond to about 70 of oxide 4) H. Goto, K. Miyazawa, W. Yamadaand K. Tanaka: amountof the steel and are mostly smaller than lOpm Tetsu-to-Hagan~, 80 (1994), 113. in size. Most of the oxides present in the molten steel 5) H. Honma,S. Okita, S. Matsudaand K. Yamamoto:Welding Res. Suppl., (1987), 301. ' before casting are also fine becausethese fine oxides can 6) R. T. DeHofi~ Quantitative Microscopy. McGraw-Hill Book not float and separate and are resultantly suspendedin Company,NewYork, (1968), 128. the steel. molten 7) ChemicalHandbook,ed. by Chemical Soc. of Jpn., MaruzenCo., (4) The oxides smaller than around lOumhave a Ltd., Tokyo, (1966), 75. large proportion of the total oxide content in the steel 8) Dictionary of Physics and Chemistry, ed. by lwanami Co., Ltd., Tokyo, (1978), 875. and govern the oxygen content in the steel. 9) lron and Steel Handbook,3rd ed., Vol. l, ed. by ISIJ, Maruzen Acknowledgment Co., Ltd., Tokyo, (1981), 15. 1O) M. Hirai, K. Kanamaruand H. Mori: Handbookof lron and The authors wish to thank Professor of NobuoSano Steel, Part l. MaruzenCo., Ltd.. Tokyo, (1981), 205. University The of Tokyo and Dr. Hiroyuki Kajioka, an l l) Solidification of Steel, Suppl. Solidification Committe, ISIJ, adviser to Nippon Steel Corporation, for their valuable Tokyo, (1970), 246. guidance and discussions in this study. l2) Handbookof Physical Propertis of Molten lron and Slag, ed. by ISJI, (1972), 6, 38.

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