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Of Oxides in Steel 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 287 C 1995 ISIJ ISIJ Internationa], Vol. 35 (1995). No. 3 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.
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