Large Inclusions in Continuously Cast Carbon Steel Billets*
By Hisashi MORl,** Nobuyoshi TANAKA,*** and Masazumi HIRAl '**
Synopsis shown in Table I by SIX strand continuous casting JlIacroscopic huge inclusions qf about IJ.05 mm or II/ore ill diameter. machine of ordina ry bending type. A tes t pl a nt Jound in the cross sections qf continuously cast 80 10 250 mm square or vertical stra nd was employed to prepa re recta ngula r 260 x 370 mm reclangular billets qf common carbon steels have been in n"stigated. T he number oj sucit inclusions decreased resjJeclil'e(v with T a ble I. Bill el size increasing sol. aluminum /Jeranlage iI/ Ihe billel, wilit decreasing to lal Bille t oJ91gen, with increasing (arbon /Jerccn lage. willz decreas ing MnJS i, with 80t.j:J 100t.j:J 11 340 145 t.j:J 16040 2 10 x 350 260 x 370 size ( mm) increasing billet si::.e. and b,v a/J/Jlimtion oj subllwged no.:zie and /JOlvder ca., til/g. The IfI~~e il/elllS ioll s ,ure JOllnd 10 collsisl, in additioll to Casti ng deoxidatioll /}fOdllcIS. of air oxidation /}foducts qf Ihe molten steel and speed 4 .03.02.3 1.7 1 . .'i 0.75 0 .55 (mJmin) SCllms entra/J/Jed into the mold. The Jormation oj large inclusions wa.1 disCllssed .from ,\lIch as/Jecls as the cleanliness of liquid steel ill Ihe CC 6 strand o rd in a ry bending I stra nd, ve rtical lundish. tlze air oxidation oj /Jouring stream, th e entra/Jlnent oj scums machi ne into tlze mOllld and tlze coagulation, and jioating-u/J of inclusions during solidification process . 350 I. Introduction The solidification of continuo usly cast carbon steel bi ll ets is cha racteri zed by the fact tha t the cra ter is narrow and deep and the directi on of solidi fi cation is
nearly tra nsversal, because o f the sma ll er section of ~40- cast billet as well as fas ter casting speed than those of (mm ) in got. 210 X 350 80 1- 14 5 1 160 1 260 X 370 For tha t reason, the size a nd d istribution o f' non meta lli c inclusions in billets a re quite different from SampleD D q those of ingot. Especially, la rge inclusions a re found I VU~ CId)J usually oftener tha n in ingots. H owever, few reports a re fo und about la rge in Sample I, 11, II' : observati on of large in clusion on polis hed cl usions in billets. This paper- is concerned with the section results of experim ent on large inclusions in carbon Sample 111 : step down test of p inhole a nd e ntra pped sc um steel bi ll ets prepa red by the 6-strand bi ll et continuous Sampl e I , II : sulfur p r int ~ macrostructure casting machine of' Yawa ta W orks of Nippon Stecl Samp le II ', if: analysis for A l a nd N B : analysis for total 0 Corpora ti on. C: analys is fo r Quant-\'ac and point counting of inclusion II. Experimental Fig. I. Sampling position Pla in carbon steels conta ll1l11 g 0. 10 to 0.85 0 0(:, 0.20 to 0. 37 0 oSi , 0.40 to 0.85 0 oMn, less than 0.030 % S, 0 a nd less than 0.030 0 P were used to prepare the test samples. H ere, let us classify the steels by C % or o Nln o, into the low-carbon steel with 0.10 to 0.25 % C, the m edium-carbon steel with 0.28 to 0. 50%C, and the hig h-carbon steel with 0. 58 to 0.85 % C, or the low ma nganese steel with less tha n 0.60 0 0Mn (MnJSi :::;; 2.5) a nd the high-manganese steel with more tha n 0.60% Mn (MnJSi > 2.5 ). Billets with more tha n 0 0.0 10 o so l. a luminum a re referred to as AI-ki lled, 0 a nd those wi th less tha n 0.0 I 0 0 (mos tl y bcl ow 0.00 5(~ 0 ) sol. a luminum as Si-kill ed . (x 8111 ) Steels were m elted in a 75 t B.O .F., transferred Photo. I. An example of very large in cl usion On a polished to the 75 t ladle, a nd cast into billets of severa l sizes cross section of a bi ll et
* O rigi na ll y pll bli shed in TelslI -ln-IIflgan!, 56 ( 19 70). IR :2 "1. in Japancse. I ~ n g li s h vcrsion r('c(' ivccl June 3, 197 1. ** 'l'edl11i cn l Resenrch Institll te. N ippo ll Sll'e l Cllf'p .. Yawaln l ron \\'1II'ks. Y:Jw:J ta-k li . K itnk), lI s 111 1 !lO:,. *** Steelma kin g Di v ision. Nippon Steel Corp .. Yawa ta Ironworks. Yawala-ku. K itakyushll eo:).
( 424 J Report Transactions ISIJ, Vol. 11, 1971 ( 425 ) bl ooms of vari ous sizes, such as 260 X 370 mm. a bsorptiometry, and sol. a luminum was ana lyzed b y Bill ets of 80 to 11 3 mm cp were cast mostl y by the magnetic mercury cath odi c electrolys is a lu minon open nozzle and oil casting process, while most of the absorptiometry. bi lI ets 0 (" 160 mm ,*, and recta ngu lar large section were cast by the ubmerged nozzle a nd powder casting III. Large Inclusions process. An inlet type submerged nozzle was used An exa mple of large inclusions visible by naked eye for prevent ing th e entrance of a ir. on the poli shed section of bill et is shown in Photo. I. Sample billets of 350 to 400 mm leng th were cut Since the small est diameter of inclusions detecta bl e as shown in Fig. I. The to tal oxygen contents of by the eye is a bout 0.05 mm, visible inclusions have bi ll ets were measured by vacuum fu sion infra red been divided into three groups: small er than 0.2 mm,
T otal 0 % 00 0 > 0.010 o 0.005 - 0.009 6 0 " < 0.004 -.J
., ~ o 0 bl) '0 ...:: o 0 u o ~. .,I- " .D E :> Z 0 _ 0 0 o cRe o 0 £J. 0 ;:: 0 " 0 '0 1- -- v ~ Fig . 2. .D ;; E Effect of sol. A I"" o n the :> ~ 2 " :> o 0 la rge inclusio ns 111 160 " o 6 "3o « m m g, a nd rccta ng ula r f- 15 20 25 10 15 20 f- 10 blooms cast with sub1l1 c rg-cd Sol. AI in bill et (I O 3%) " nozzle a nd powder ~ AI < 0.005% l ~~ 3 I- U o '0'" Sol. AI 0.006 - 0.0 10% "'0 L6 Sol.AI > 0.011 % -0 E '::- ~ ~ E E o u I- Report ( 426 J Transactions ISIJ, Vol. 11, 1971 0.2 to 0.5 mm, and larger than 0.5 mm in diameter. of total 0 % of bille ts. As shown in Fig. 3, the number of large inclusions i . Effect of Various Factors on L arge inclusions in 160 mm cp and rectangular blooms increases pro 1. Effect of Alutninurn, Oxygen, Carbon , and Mn/Si Ratio portionally with increase of total 0 % . As shown in Fig. 2, the number of large inclusions When total 0 % is equal, the number of large in in 160 mm 90 and rectangular blooms decreases hyper clusions decreases with increase of sol. Al %. bolically with increase of sol. Al % . The effect of' As shown in Fig. 4, the number of large inclusions so l. Al % is especiall y marked in large inclusions of' in open nozzle Si-killed 80 to 11 3 mm 90 billet decreases over 0.2 mm diameter. In case so l. Al % is equal, hypel'bolically with increase of C % in billets. the numbel' of' large inclusions increases with increase F igure 5 shows the effect of C% in steel on the average number of inclusions in Si-killed billets, which No. I and No.6 strand a re classified by C %, Mn%, and billet size. The 40 number of large inclusions is larger in the low-carbon steels than in the high-carbon steels, except the low N 30 E manganese steels o f 80 mm cp . u 0 20 As shown in Fig. 6, the number of large inclusions 0 .. ' .:::: lJl open nozzle Si-killed low-carbon 80 to 11 3 mm 90 10 VI billets increases with increase of MnjSi. It is shown c: 0 .iii 0 in Fig. 7, replotted from Fig. 5, that the number of inclusions is larger in the high-manganese steels than U" c: lJl the low-manganese steels. This increase of large 60 OJ) ....'" No.3 and No. 4 strand 50 VI 60 '" c: o No. I and '0 0.4 - 0.6%Mn 0 VI No . 6 strand s .... 40 0.6 - 0.8%Mn 50 ,r,'" U" ' No . 3 and E c: 30 , .' No.4 st rand s "c: "' ~ 40 biJ N ;0 E " '" 20 u '0 - 0 30 f- ~o o ~ lO .... " 0 "' ~ , .D 20 r ',' ~ o .. E :~ ; : 00 " 0 0.100.20 0.300.400.500.60 0.700.800.90 c: , 0 0 , " 10 • 0 C% in steel ~ io i~ .~ :r~o l ~ '0'" ~ 00 ' § . 0 '1 0 0 f- 0 8 F ig . 4 . Effect of Co 0 in steel on the total number o f large 1.5 1.7 1. 9 2.1 2.3 2.5 2.7 2.9 3.1 inc lusions in Si-ki ll ed 80 to 113 mm 90 billets cast Mn l Si in s tee l wi th open nozzle F ig. 6. 80 mm l/ 96 - 100 mml/I 11 3 mm i 160 mm l Rectangu lar 0.4 - 0.6%Mn Effect of Mn/Si in steel on the 10 total number of N large inclusion in E u Si-ki ll ed 0.1 to 0.2° o C 80 to 113 "" ...... " "- "" .. mm cp billets cast Dia . of VI c: inclu sion with open nozzk 0 0. 1 - __ A , o < 0.2 mm VI &- - -- , , · 0.2- 0.5mm u" , .. > 0.5 mm c: , , biJ 0.01 , 0.005 0.007 '"... A ______...... '" 0 - ~ '0 0.6 - 0.8%Mn .... 10 "------., ,r,'" E "' "c: ..-- .... , " '0 , .. '" , - ...'"''" .. '" <'" 0. 1 ... ------..... Fig. 5. 001 Effect of Co;" in steel on oT J } } ! ' ... the averaged number of 0.1- 0.2 0.6 - 0.85 0.1- 02 0.6 - 0.85 0.1 - 0.2 0.6 - 0.85 0.1 - 0.2 06 - 0.85 0.1 - 02 06 - 0.8 5 large inclusions in Si C% in steel killed billets Report Transactions ISIJ, Vol. 11, 1971 ( 427 ) SO mm l 96 - 100mm l 11 3 mm / 160 mOl .' Re c tang ul a r 0.1 - 0.2%C 10 ~ ~ ./ E ./ u <=> /' <=> ./ '" 0" 0.1 '"::0 U .:: , " V bI) 0.01 L ~:", // s- 1 ~ O 0005 0.007 '0 0.6 - 0.S5 %C s- V 10 .D E ::0 -0" V bI) ... - --- V'" «:> - --- 0.1 Dia. of 4 ------_ .lo. inclusion 0 < 0.2mm .. • 0.2- 0.5mm Fig . 7 . .. > 0.5mm o 0.0 1 I Effeet of 1\1n 0 in steel on the 1 1 t J I I .. ------.. } averaged number of large inc lu 0.4 - 0.6 0.6 - 0.8 04 - 0.6 0.6- 0.8 0.4 - 0.6 0.6 - 0.8 0.4- 0.6 0.6 - 0.8 0.4 - 0.6 0.6 - 0.8 sions in S i-killed billets Mn % in s tee l 0. 1- 0.2 %C 0.6 - 0.S5 %C 0.4 - 0.6 %Mn 0.6 - 0.S %Mn 0.4 - 0.6 % ~ l n 0.6 - 0.S %Mn ' ~ f in clu sion < 0.2mm '" · 0.2- 0.5mm o" > 0.50101 'Vi I' ::0 10 , ~ u " ~ ~ .. I ' ' .., /', ~ , \ /\ , /" y \ ' V \ ,, \ \ , \ \ , 1 0.1 /\ " r \, ~\ l ," " \ ,l' , , , j O.oJ ' "', '. 1 '. , ,, \ , ' " Fig. 8 . .< oI ;~~o5--- ooo d I I 'i '. 1 lo' ~8~0 ~96~-~1 ~13~1~60~-+\~~80~9~6-~11 ~3~16~0 --'-~8~0 7,96~-~11~3~16~0---'~8~0~9~6-~1~13~1~6 0~-+~ Effec t of billet size on the averaged 100 Rectangu la r 100 Rectangular 100 Rectangul ar 100 Rec tan!(ular number of la rge in c lusions in Si-ki lled billets Bi ll et size ( Olm ) inclusions with in crease of Mn/Si is in contradiction bi ll ets, the number of large inclusions (over 0. 5 mm in to the fa ct tha t the number of entrapped scum de diameter) being only 10%. Rectangular blooms have creases with increase of Mn/Si.1 l a higher incidence of large inclusions than 160 mm g, 2. Effect of Pos ition of Strand billets. As will be noted from Figs. 4 and 6, the No. 3 Figure 9 shows the effects of billet size on the num ber and No.4 strands, which are nearer to the pouring of large inclusions in AI-kill ed billets having over stream from the ladle, show a larger number of large 0.01 % so l. a luminum. In the case of submerged inclusions in Si-k ill ed 80 to 11 3 mmg, bill ets, than the nozzle and powder casting billets, it is seen that the No. I and No. 6 strands, which are farther from th e effect of bill et size is not clear in the range of 100 mm y:, pouring stream. to rectangular large section a nd is different from the 3. Effect of Billet Size case of Si-ki ll ed billets. For inves tigation of the effects of the size of Si -ki ll ed 4. Effect of Submerged Nozzle and Powder Cas ting bi llets, Fig. 4 was replotted to give Fig. 8, which shows T he total n umbers of large inclusions in Si-killed that the number of large inclu ions in 160 mm g, 100 and 145 mm g, billets and of AI-ki ll ed 100 and 113 bi ll ets is a bou t 30 to 50% of that in 80 to I 13 mm g, mmg, bil lets, as shown in Fig. 10 (compared in the Report ( 4 2 8 J Transactions ISIJ, Vol. 11, 1 971 12 Dia. of Nozzlt' Open ""'" c% 12 inclusion E;;-' 0.3 < 0.2mm E E 10 0.45 u > 0.6 . ~--- 11 0.2 - 0.5mm 00'" i > 0.5 mm /\ S 8 N 10 ~ " E OJ ~ u .... 0" 0 9 _ '""" . Report Transactions ISIJ, Vol. II, 1971 ( 429 J 111 80 mm g, open n ozzl e iJill ets increases with increase because contamination in the tundish was ej ected in 111 the a mount of powder added . When the a mount the early stage of casti ng or because disturbance of of syntheti c slag powder added is equal, the numbcr th e po uring tream into the mould was strong before of large inclusions increase rema rkably with a lower the head of the mol ten steel 111 the tundish reached melting tempera ture or a lower viscosity of the powder. the stead y level. 7. Effect of Titanium Addition in Ladle The a mount of entrapped scums in Si-killed , me As shown in Fi g. 14, the number of large inclusions dium or hig h carbon, and low MnjSi billets decreases in 80 mmg, billets increases in the high-ma nganese with inert gas seal of the stream pouring into the steels, but decreases in the low-manganese steels, with mould a nd increases with bl owing of air into the increases in the a mount of tita nium added in the pouring stream. 11 In low MnjSi 80 mm g, medium or lad le. This i in contradicti on to the phenomenon high carbon steel billets, however, scums were hard that the number of entrapped sc ums in low-manganese and the number of large inclusions decreased . Then steel s increases with addition of titanium in the ladle. 11 the effect of i nerL gas seal or bl owi ng of ai r into the 8. Other Observations pouring stream was obscure, and the numbel' of large The change in the number of large inclusions during inclusions was, in each condition, about 3f1 00 cm 2 • casting was not recognized. In the very early stage Bes ides all the factors d escribed a bove, multiple of casting, however, billets with many large inclusions correlation a nal ys is revealed the tendency for the were occasiona ll y obta ined. This was probably number of la rge inclusions in Si-killed 80 to 11 3 mm g:, open nozzle billets of a ll size to decrease wi th increase of the time between ta pping a nd casting, a nd for the eI; - 50 ~ '",- N • Mn > 0.6% number of la rge inclusions in 160 mm g, a nd recta ngular " E - u 40 L' Mn < O.6% ~o bl ooms of submerged nozzle a nd powder casting (ovel' 0 0 '- 0. 2 mm dia meter) to d ecrease with increase of the 1! - 30 E e length of columnar d endrite, i.e., with increase of 0 20 "c "V; the casting scuperheat tempera ture. ~u" 10 I-" .-e : g . ~ 0 I ~ 0 2. Siz e D istribution alld Trallsverse D istributioll of L arge 100 200 300 Inclusions Amount of AI · wire add ed In mould ( g/ t ) 1. Size Distribution Fig. 12. Effect of A I-w ire addition o n large inc lusions in 80 to 11 3 IIl1ll y:, bi ll e ts (0 .55 to 0.85°,, (; steels) By repl otting Fig. 5, the rela ti onship between to ta l number a nd ra ti o of over 0.2 mm or over 0.5 mm e diameter to tota l number of la rge inclusions in Si-kill ed 0 'in SO bi llets is shown in Fig. 15, from which the following u" can be d eri ved : e ~" (a) The ra ti o of la rge sized macro inclusions '""< - 60 ~ Ne increases with increase of the MnjSi in steel. u ~o (b) The number of large inclusions increases with 0 0 40 :... ~ increase in the total number of inclusions. 1! - E (c) Despi te the presence of fewer la rge i ncl usions "e 20 111 160 mm g, and rectang ula r blooms tha n in 80 to ~. '" gg, I- :I J SO- 11 3 mm . 160mm and " ~ 200 300 400 3 0 100 with open nozz l e r ectangular with . Am ount of powder add ed in mould ( I' t ) s ubmerged nozzle Fig. 13. Effect of p owde r a dd itio n o n la rge inc lusio ns in Si and powd ·e r killed 0.6 °,, (; 80 10 11 3 mm cjJ b illets cast w ith ° ~ open nozzle . 0 ~ VI 0.63%C e 40 0.63 %C o.S%Mn/ I 0 ~O..75S%%MnC VI 0.5%Mn o I. I. Mn %C%1 < 0.2 > 0.6 v" / e I ' 30 / I . . 0.4 0.6 0 ~ 3 0.6 - 0.S" '" I I - . - '-"" N~ / I · ~ E u 20 I I ~o 2 0 I 0 / / "''-~-..... -:.... -0 - -.,.. I 0 E 10 . . "e .~ --- :--- -;;; , --- · 0 0:::'" 0 . 0 o 5 10 15 20 25 0 1 2 3 4 5 6 20 20 I-" 0 10 15 20 0 0 T otal numbe r of large inclus ions Amount of Ti add ed ( kgl75 t of s teel ) (f 100cm') Fig. 14. Effect of Ti a dd itio n on the to ta l num be r of la rge Fig. 15. R e la tio n be tween total numbe r a nd ra tio of very inclusions large inclusions (Si-killed billets) Report ( 430 ) Transactions ISIJ, Vol. 11, 1971 11 3 mm cjJ bi ll ets, the number of large inclusions over Low ma nganese steels a bounded in Si0 2-rieh in 0.2 mm in diameter is found almost equal for a ll clusions with a n inner refle cting ring and inclusions classifications. in which Si02 has precipitated showed a high I-ate of 2. Transvers e Dis tribution Aw ay from Skin preclJ)Jtation. While the effect of C % in steel is The number of large inclusions in 160 mm g, a nd smaller tha n that of Mn(Si , high-carbon steels, be- rectangular blooms, distributed tran versall y away from the sk in, was as shown in Fig. 16, g reates t in the A I,O% axia l center in the case or Si-killcd low carbon billets Sol. AI > O.O I o Sol. T.O < O.OI -· with more than 0.01 to ta l oxygen content. 13 % L AI< O.O I ~ T ~> 0.01 0-- --·" E 12 The number of large inclusions in excess of 0.2 mm u <:> II in d iameter tends, it is seen, to be large in the skin a nd <:> in the axia l center and small in the intermediate zone, .:::::: 10 especia ll y it is the greatest in the axia l center in the case of Si-killed billets with over 0.01 °0 total oxygen content. o " ;;-... . ------,," .-"-., 3. Classification and Composition oj Large i l/clusions 0'" ---0 1. Large Inclusions in Si-killed Billets Microscopic view or typical large inclusions a rc given in Photo. 2. All are almost spheri cal Mn sili cate structures, regardless of the carbon content OLI__ -J ______-L______L __~ ______~ ____~~ in steel, which can be classifi ed into three groups: the first has a strongly refl ecting ring insid e; the second 2[ 160mm ' 00 Rectangul ar bloom is single-phase, the ring being a lmost invisible; and E _ E1: the third has a flowery pattern of precipitated Si0 . 2 '" u Table 2 shows the composition of la rge inclusions ~ g V''::::: I a na lyzed by EPMA. The ring-shaped in clusions ~ ~ ~ c - 0 are of Mn-sili cate which contains much AI 20 3 and - "Vi o " . Si0 2 Figurc 17 shows the rela tionship betwecn thc " u U c kind of the inclusion and the composition of liquid steel. r- From the range of occurrence of Mn-sili cate marked z Skin ~ I idd l e Cenlral S kin Middle Cenlral by Si0 2 precipitation, the experimental reo ults seem to be closer to those of Korber and O elsen 21 than to Fig. 16. Transverse distribution of large inclusions in bi llets those of Nagano et af.3 1 cast with submerged nozzle and powdcr (x 100 ) ( X 4 ' 7) ( X 400 ) ( X 4!7) Photo. 2. Large inclusions in a Si-killcd billet (a), (b) Single pha~c I\ ln-silicate (c) Mn-sil icate with c rys toba li tc prccipitates (d) t\[n-s il icatc w ith corrllncllllll precipita tes Re port Transactions lSI], Vol. 11, 1971 [ 431 J Table 2. Compositi on of large inclusion by EPMA Composition of inclusion Composition of billet Bille t Pho to. size Condition of deoxidation and CC No. AI 20 a Si02 MnO FeO CaO Ti02 C Si Mn T.AI ( mm) 2- a M atri x 22 47 25 1.0 3 . 0 I . 0 O. I 7 O. 28 O. 50 145g, Si-ki ll ed , open nozzle 2- b Matrix 8 45 47 0 . 1 0 .5 0 .74 0.28 0.79 80g, 2- c M atrix 1.0 38 65 0.2 0 . 17 0.36 0 . 75 100 g, M atri x 36 39 14 10 1.0 2- d 0 .73 0.24 0.49 80g, ppt 99 0 .2 Matrix 1.0 39 60 0 . 2 0.3 0.71 0.36 0.75 IOO g, M atrix 34 33 31 0 .8 3- a 0 . 17 0 .28 0 .64 0.007 113g, Si-killed , Al wire 50 g/t ppt 98 1.1 0 .7 Matrix 36 39 14 0 .5 10 1.0 3- b 0.82 0.21 0.82 0.006 80g, Al wire 50 g/t ppt 99 0.2 0 .5 0 . 1 3- c M atrix 70 15 1.1 2 .6 0.64 0 . 25 0.53 0.010 96¢ Al wire 100 g/t 4 M atrix 15 44 32 < I 5- a M atrix 5 1 22 0 .6 1.8 25 0 .63 0 . 24 0.52 0.007 11 3g, Si-killed , powder 80g/t Al wire 100 g/t M at ri x 10 5 0.3 0.8 83 5- b 0.62 0 . 29 0.53 0.01 I 11 3g, , powder 40 g/t Al wire 100 g/t Matrix 38 5 0.2 0.5 7 M a trix 42 7 58 0.62 0.29 0 .53 0.010 113 g, , powder 40g/t Al wire 100g/t M a trix 28 40 24 1.0 5 0.5 6- a 0.46 0.25 0.78 0.025 260 X 370 Al killed , submerged nozzle and powder ppt 99 o 0.3 M atrix 65 39 6- b < I sometimes found to contain granular iron. However, the Al 20 3 content of large inclusions in the same b il let frequently showed large differences. Also while large inclusions were of Mn-silicate struc tures with precipitated A1 0 , the majority of the 0.60 2 3 inclusions less than 50 fl in the same specimen consisted 5 frequently of Al20 3 onl y. Moreover, CaO was de 0.40 tected in large inclusions composed mainly of A120 3 • These fa cts suggest that some of the Mn-silicate in clusions which have been contained in the pouring .~.7::° < ~ : . ~ ' 1500'- 1520 ' C 0.20 Si02 satur ated 3) bath or have been fonned by air oxidation of the Mn. Silicate 3 ) pouring stream reach equilibrium with the composition of liquid steel, whereas others do not, depending on 0.40 0.60 0.80 1.00 the length of time required for solidification as well Mn % as on the size of inclusion, in their course of becoming progressively reduced by aluminum added to the Fig. 17. R elation between the kind of incl usions and Mn and Si contents in Si-killed steels mould in the form of aluminum wire, and that both remain and coexist in the steel. cause of the higher yield of aluminum contained in 3. Large Inclus ions in Billet wit h TitaniuDl Added in Fe- Si, etc. compared with low-carbon steels, some Ladle times had inclusions with Al 20 3 precipitated into Mn Large inclusions in billets with 0.13 to 0.27 kgft silicate. of titanium added in the ladle have, as shown in 2, Large Inclus ions in Billet with AlurninuDl Wire Added Photo. 4, a feather-like pattern of Ti02 precipitated in Mould on the sili cate matrix, the amount of precipitation Photograph 3 gives typical microscopic views. in creasing in proportion to the amount of titanium G enerall y, with greater a luminum wire additions, the addition in the ladle. content of AI20 a in Mn-sili cate increases, and then 4. Large Inclus ions in Ope n Nozzle Billet w ith Powder the precipitation of corrundum in Mn-silicate in Added in Mould creases. Furthermore, there occurred amorphous As shown in Photo. 5 and Table 2, large inclusions, or cluster-shaped inclusions containing more than when powder and a luminum wire were added in the • 80% AI20 a. These Al 20 3-rich large inclusions were mould, contain much CaO a nd A1 20 3 Considering Report ( 432 J Transactions ISH, Vol. 11, 1971 - .. " .: , , • l (X 100 ) ... ( X 47l : ( X 400 ) (X 4 7) Photo. 3. Large in c lusions when AI-wire was added into mould (a), (b). and (c) AI,03 rich Mn-silicate with corrundum precipitates (d ) A,O" cluslcr • ( a ) (X 100 )( X 2/ 3 ) ( X 400 )(X 2/ 3) Photo. 4. A large inclusion in a Ti deoxidized billet the composition of the powder, the above phenomenon seems to be caused by the powder being entrapped In the inclusions. 5. Large Inclus ions in AI-killed Billet Typical m icroscopi c views are shown in Photo. 6. I n most cases, corrundum precipitated in the Mn sili cate matrix and Al 20 3 clusters are recognized. In the case of open nozzle billets, however, plain Mn si licate is sometimes found . ( b) When a Ca-based deoxidizer was added in the (X IOO )(X 2/ 3) ( ><4 00 ) (x 2/ 3) ladle, inclusions accompanied by CaO and A1 20 3 Photo. 5. Large inclusions when powder and AI-wire were remained as clusters or large inclusions. added into mould Report Trdnsdctions ISI], Vol. 11, 1971 ( 433 ) .. • ' ~ .. .;.,..; . ."E .:;• .1* .,.... ( X 100 ) (X 417) (x 400 ) (b) (x 4 7 ) Photo. 6. Large inclus ions in an I\I-killed bi llet (a) A I20" rich l\ln-si li cate w ith corruneiulll precipilates (e) CaO rich ,\1 ,0 " c luster (b) /\1 ,0" I CaO (d ) l\ln-sili cat(· 6. CODlposition of Large Inclusions The compositi on or large inclusions in Si-killed .~" 1.4 r billets, typical examples being given in Table 2. was " 0 0 0 g 1. 21 o 38 to 50 0SiO", 20 to 60 0MnO, 2 to 40 0A1 20 a. '" , Co mpn . of ~ The content of AI20 a or Ti02 increased w ith increasc " 1.0 mou ld scum of a luminum or titanium additions. As shown in Fi g. 18, the ratio l\lnO/Si02 in large inclusions in Si o 0.8 killed billets increases with in crease of l\fn/Si in the 0 steel, a nd at the level where the Mn/Si ratio is equal, o 0.6 1 o 0_ 0 C% .! < 0 , 25- 0 ' 25 - 0 ' 55 ,>'0' 5~ 1 the MnO/Si02 of large inclusions in submerged nozzle 'fl . ~ 80 - 11 3 ' 0 6 0 160 mm tj:J and rectangular bl ooms is lower than that ~ 0.4 160 • ... • ~ :;:;: ~ r('ctanf{ular ~ in open nozzle 80 to 11 3 mm ,*, bill ets. The MnO/ 1.5 2.0 2.5 3. 0 3.5 4.0 Si0 or scums in the mould was higher than that of 2 Mn Si ratio in s t eel large inclusions. When a Ca-hased deoxidizer was added in the ladle, Fi g . Ill. R ela ti o n be tween l\ln/Si ra ti o of stee l a nei l\LnO/ SiO, ra ti o o f large in c lusions there were d etected 2 to 40° oCaO in the large in clusions a nd 2 to 15° CaO in the sc ums. Since the o point counting method, a ll inclusions were classified content of CaO in plain Si-killed billets was less than 0 as follows: 1.5°0 in the sc ums and 0. 5 to 10 0 in the large in clusions, it was assumed that primary deoxida ti on AlzOa products were contributing to the origin of large in Silica cl usions. Silicate Oxides J Considering that 0.2 to 1.5% M gO was contained Silicate + Al zOa precipi lates in the scums and tha t, upon addition orglass containing Sulfide+sili cate 6(Yo Na to the tundish, about 0.2 °o Na was detected 1 Slfide+ AlzOa in the scums, it i · reasonable to assum e that furnace T ype i slag and tundish slag are among the sources or large Sulfides f T ype 1/ i ncl Ll sions. \ Type iff IV. Cleanliness of Inclusions Photograph 7 illustrales typical examples of in clusions. The oxide inclusions observed include, in the I . (,'lu.I".I"iji("ulioll of /III"/llsioll.\· case of Si-killccl hillets, , ili ca, sili cate, sili catC' + i\1 20 a III nlicroscopic examination of in elusio l1 s by the pi rci pi la les, a nd sulfide +sili cate; in tlte case of AI -killed Report ( 434 J Transactions ISIJ, Vol. 11, 1971 • .... (a ) (b) ( e ) (d) (e ) .» '., ... ( X 400 ) (f) I (g ) (h) ( i ) ( X 7/ 10 ) Photo. 7. Examplcs of inclusions (a) AI 2 0 3 (d ) Mn-silicate with AI 2 0 3 precipitatcs (g) Sulfide (type I) (b) SiO, (e) Sulfidc + Mn-silicatC' (h ) Sulfide (type I I) (c) Mn-sili catc (f) Sulfid e + AI ,O" (i) Sulfi de (type III ) 0.150 0.125 S ki n pa rt ·-----' 0.100 S kin part 0.07 5 0.125 0.050 - 0.100 0.025 ~ 0.075 0 0" * 0.050 '" 0.150 u" Middle part 0.025 0.125 o " .iii " 0.100 OL-__-L __ L-~~ __~-L ~~ ."'" U" >< 0 0.07 5 .= ~ 0 0.050 "0'" X 0" o EJ 0.025 ~ Cent ral part u o '"... 0 0.175 .S" '" 0.150 '"... ~ 0.150 Report Transactions lSI], Vol. 11, 1971 ( 435 ) d ecreases parabolically with increase of AI% . The ladle, tundish, nozzle, stopper, etc. ), air oxidation pro effects of (:00 a nd so l. Al oo a rc essentially dependent ducts at the su rface of pouring stream and molten on total 0 °0 ' steel, t" apping of scums in the mould by pouring stream (deoxidation products in the mould), etc. 3. Size D istribution and Trallsl1e rse D istribution oj i n Parenthesized are the kinds of so urces, for which no clusions Away Jrom Skin direct experimental proofs have so far been obtained. As shown in Fig. 21, t he rate of la rge inclusions of For discuss ion of the effect of various origins and 10 to 50/-t is higher in Si-killed billets than in AI-killed factors on the number and distribution of large in billets. The a rea fraction of large inclusion a nd the clusions in CC billets, it will be suitable to examine area fraction of all incl usions, in Si-kil1ed low carbon in sueees ive order, as in case of the steel ingot process, billets, are larger in the transverse directi on away fr om the cleanliness of molten steel poured from tundish, the skin than in the skin. This corresponds with the the contamina tion of pouring stream due to air oxida distribution of la rge inclusions discussed before. ti on or the like, a nd the solidification process. V. Sources of Large Inclusions J. Cleanliness oj Pouring Molten Sleel From the results d escribed above, the following F igure 22 shows the cha nge of total 0 % from in may be pointed out as the sources of large inclusions: the ladle, tundish (both by means of the bomb sam converter slag, (slag in the ladle), slag in the tundish, pling), to billet, campared with the equilibrium oxygen d eoxida ti on products in the ladle, (refractori es of the values Q in the Fe- Si- Mn- (AI )-O system as calcula ted from the data of Walsh el al. ,4) and the values recom mended by Gakushin (Science Promo ti on Association, Inc lu sion s ize Japan) .5) :: ; < 10,u u . ll - 20 ,u If the difference between tota l 0 % and Q% equa l ' E ~ 21 - 50,u the a mount of oxide suspending in the molten steel, ...... ~ 0.05 o ~ the amount of oxides in Si-killed molten steel should 0.04 o" 0" increase with decrease of (:00 in the steel. Figure '.:: ';:;; u " 0.03 23 shows the relation between C % in Si-ki ll ed steel "'-... u .... " 0.02 a nd number of large (> 50/1 in dia m eter) inclusions ...'" 0.01 in the bomb samples taken at the ladle or tundish. < 0 Both in the ladle a nd in tundish, the number of in 2 clusions ( >50 ,1/ ) increases with d ecrease of (:0 0 in Sol. AI > O.Ol % Sol. AI < 0.005% steel. ( I ) Skin part (2) Middle parl (3) Cen lral part Consequently it is hig hl y probable that one reason Fig. 2 1. Transverse a nd s ize distribution of mi c ro inclusions why the number of large incl usions in Si-kill ed billets ( 160 mm cF and reclangular blooms caSl w ilh sub in ueases with decrease of C oo res ts with the large merged nozzle- and powder) amoul1l of ox ides suspending in molten steel in the .. 0.25". 5i 0.5'. )l n 0 eq uilibrium h II" I h -- O.25 <1l1 Si 0.75 (i~,\l n 0 efluilihr ium ~ a s - O.60 Q'O C 0 eQ u i I ibr ium O.O llill AI O('(lu ilihr iu m Si ·kil led s leel ; AI·kill ed s l eel J 0.1 - 0.2 %C 0.3 0.5 %C 0.6 0.85 %C Noz- S ub - C% zle Open me rged 80 - 11 3 0101 250 ~0.30 - 0.50. A .. • 160mm c' 0.60 - 0.85 • Rectangu lar .1: ladle a t just 200 a fl e r lapping B : ladle at rest E c- on lhe cast- o. 150 00 . ing fl oor o c: lund ish .: 0 =~ o .O.,O,?, ~ • i 00 0 1 : billel w ilh ~ 100 00 r , 0 0 co op en nozzle , 00 • OCX) 000"" .. ..0 , ~~~~~o~~~ __ 0, : bill el wilh ' \~ : . .. + ';1 ~ 6 : subme rged 50 ' "--._ 0,' ..001::0 • 0 o • A ~o nozzle and powder Fig. 22. A o /J C J) 2 A /3 C D2 A /3 B T e mpe ralure C c C ha nge of lOla l of mol ten 16 50 1635 1555 1625 16101 530 1595 1580 1500 oxygen from ladle s l eel (O e ) to billet Report [ 436 J Transactions ISH, Vol. 11, 1971 ------0.2 5%S i 0.5% Mn · Q eQUilibrium l I E <.) 70 -- 0.25%S i 0.75%Mn -Q equilibrium W'l ' bJ co In ladl e ill cas ting fl oor _.- O.6%C Q equilibrium 60 25 <.)" 20 In lundi sh ~ 0- - 0 _ _ • ~o ~- °0 - • -- o " f- -.....""",,- o 0 0\. " co on 15 !\ ~ '0 ..<: 0 0 ,., • 0 5 '" 10 0 . Report Transactions ISIJ, Vol. 11, 1971 [ 437 ) addition of tita nium in the ladl e. This is proba bl y kill ed billet size is cha nged fr om 100 to 160 mm cp , because sc ums in the mould a loe more easil y d riven but in thc steel ingot process the number of large a nd dispersed into the molten steel by the po uring inclusions in creases with in crease of the killcd steel stream th e lower the melting point and the lower the ingot size. T his a ppa rent contradicti on can be ex viscosit y of the scums in the li q ui d steel. H owever, pla ined as foll ows. even in case of 160 mm ,:? a nd recta ngular Si-killed J n the steel in got process the secti onal diameter is blooms with submerged nozzle, the influence of Mn/Si in the order of 500 to I 000 mm, a nd the ingot hig ht is recogni zable. This reveals that the Mn/Si ra ti o is in the order of 2 to 3 m . During the initia l peri od influences not onl y the entrapment of se ums but also of solidificati on when columnar crys ta ls solidify, la rge the behavior of inclusion during solidificati on. inclusions in rema ining molten steel fl oat-up to the hot top by the acti on of fluidity by thermal convec 3. Solidification Process ti on throug hout the remaining steel. 9 ) An appm x First let us con ider the influence of steel compos i imate calcul a ti on based on Stoke' law indicates tha t tion. As will be disc ussed later, large inclusions are the time required for fl oating a way from a depth of not expected to fl oat-up during solidificati on in sm a ll 3 m up to the surface of molten steel is 3.S min in the section billets, therefore it is probable that the n umber case of large in cl usions having a diameter of 200 I I a nd size of large inclusions rema ining in the billets a nd 0.9 min in the case of 400 ,lI . This means tha t will increase in proportion to the number of oxygen in cl usions can satisfactoril y fl oat-up d uring the period sources and to the coagulating and growing property of colu m nar crys ta l forma ti on. When solidificati on o f incl usions. proceeds further to the peri od of forma ti on or equiaxed I n silicon deoxidation, the limi t of deoxidation is grains, therma l convection is preven ted in the so much dependen t on temperature, a nd a large a mount call ed sedimentary zone, i.e., a sedimenta ti on layer of seconda ry deoxidation products forms during solidi from the bottom , so tha t the form ed in clusions a nd fi cation, 0 tha t these will coagul ate and grow with Q a re tra pped in the sedimentary zone a nd reta ined oxides suspending in the po uring molten stecl to in a so lid liquid coexisting phase, therefore they become la rge incl usions. The num ber of la rge coagul a te a nd grow10 ) in to so-call ed sedimenta ry zone inclusions in creases wi th in crease of the to ta l 0 ° 0 la rge in clusions. For the same compositi on a nd in Si-killed bil le ts, p robably because of the a bunda nce cleanliness of pour ing molten steel , these sedimenta ry of oxygen so urces. The number of la rge incl usions zone large in clusions increase in number a nd size with increases with decrease of the C oo in billets, because increase in thc solidificati on period for equiaxcd the amoun ts of oxides in the tundish a nd of air oxida gra ins i.e., with in crea e in size of the ingot. tion products in the pouring stream in crease with On the other ha nd, as features of the CC process d ecrease of Co 0 ' Consid ering tha t the equilibrium covered in th e present experiment, the foll owing Q valu e is little influ enced by Coo, the influence or may be cited : (a ) thill so lidification thickness a nd the a mount of seconda ry deoxida ti on products during short solidificati on peri od ; (b) na rrow a nd deep solidificati on will not be importa nt. cra ter, nearl y representing tra nsverse so lidification ; Also in the case of billets with submerged nozzle and and (c) fast ca~ tin g speed . An a pproxima te calcula powder casting, hi g h ma nganese steels show a la rger ti on based on Stokes' law indicates tha t the critical number of large inclusions, pres uma bl y because in size of inclusions capable to fl oat-u p again st casting clusions with hig her Mn/Si02 ra ti o in high-ma nganese speed is more tha n 400/, in the case of SO mm cp billets steels can coagulate easil y. Thermod ynamicall y, in with a casting speed of 4 m /min, more tha n 250 II clusions with hig helo MnO/Si02 ratio a re expected in the case of 160 mm ,:? with a casting speed of 1. 5 to have less coagula ti on a bility, because in cl usions with m /mi n, a nd more tha n 150 to 200 fI in the case of hig her MnO/Si02 has sma ll er in terface tension reetagul a r bl ooms with a casting speed of 0.55 to between in cl usions a nd molten steeP) I n hig h 0.75 m /min. This means that the fl oating-up of m anganese steels, however, increase of the M nO/Si02 large in cl usions becomes increasingly ha ndicapped ratio of i ncl usions wi II cause decrease of m el ti ng with d eCl"ease 0 (" the bill et size. The number of la rge tempera ture a nd viscosity to res ult in im provem ent or inclusions is markedly sma ll er in 160 mm ,:? bille ts coagula ti on. tha n in SO to 11 3 mm g, bill ets. This is proba bly The n umber of la rge inclusions ra pidly decreases because the effect of facilitated fl oatati on of la rge with increase of so l. AI % in billets. This is because incl usions has over shadowed the unfavora ble effect tota l 0 % in molten steel and billets is low and sec of accelera ted coagula tion and growth of large in onda ry deoxida ti on products d o not form, therefore cl usions by the enla rged of billets. inclusions cannot grow. Furthermore, it will be a n The number of la rge inclusions is slightly greater important fac tor tha t the coagul a ti on a bility of in in recta ngula r blooms than in Si-killed 160 mm ,:? cl usions cha nges, i.e., ha rd sili cate rich in AI 20 3 cluster bill ets. In recta ngul a r and 160 mm cp Si-killed low appears. carbon steel billets, the number of la rge inclusions Next, the difference of solidification conditions having a dia meter of more than 0.2 mm is greater between the CC process a nd the steel ingot proces in the axial center tha n in the m iddle part. This a nd the influence of billct size will be discussed . leads us to assume tha t solidificati on in the axia l It is wcll known tha t in the CC process thc n umber center, similar to tha t in the sedimen tary zone under of la rge in clusions marked ly decreases when the Si- the ingot process, prevents the fl oating-up of inclusions Report [ 438 J Transactions lSI], Vol. II, 1971 or Q by entrapping them in the olid- liquid mushy the mould and, the amount of entrapped scum, zone and thus favors their coagulation and growth. i.e. with increase of Mn/Si in steels or addition of This assumption is in accord with the fact that the titanium to high manganese steels, means that the number of large inclusions with a diameter o f more entrapping rate of scums increases wi th such variation than 0.2 mm increases with d ecrease in the length of of the scum properties. columnar crystal in l60mm ~ and rectangular blooms. Therefore, it can be expected that the number of 3. Solidification Process large, es pecially very large inclusions, increases fur The number of large inclusions decreases with in ther when the billet size becomes larger than that of crease of sol. Al% in the billets. This is explained rectangular blooms under the CC process, too. by assuming that large inclusions containing AI 20 3 as principal component cannot easi ly coagulate and VI. Conclusion a lso that they do not grow because secondary deoxida The effects of several fa c tors on the formation of tion products do not form during solidification. large inclusions in continuously cast carbon steels and Also in the case of billets with submerged nozzle and their action mechanisms may be summarized in the power casting, the number of large inclusions is greater order of the process as follows: and the percentage of large-size inclusions is higher with increase ofMn/Si in billets. This will be ascribed 1. Cleanliness M olten Steel in Tundislz of to the fact that MnO/Si02 increases with increase of The number of large inclusions in Si-killed billets Mn/Si in steel, the melting point decreases, and increases with decrease of C o ° in steel becau e the coagulation becom es asy. amount of oxidation products suspending in molten In the case of Si-killed bill ets, the number of large steel in the tundish increases corres pondingly. The inclusions is markedly less in 160 mm g, billets tha n amount of oxides suspended in molten steel in the in 80 to 11 3 mmp billets and sli ghtly greater in rectan tundish as the source of oxygen for large inclusions is gular blooms such as 260 X 370 mm size than in 160 equal to or more than the amount of secondary deoxi mmp billets while in blooms with large cross section dation products given birth to during solidification. the number of large inclusions is greater in the axial The number of large inclusions d ecreases with d ecrease center than in the middle pan. This sugges ts that of the total oxygen content in billets. there is an optimum size for the section whereby large Since MgO is detected in scums in the m ould and inclusions are minimized in the synergism of two ef with addition of glass in the tundish natrium is detected fects, one being positive in that the Ooatating-up of in sc ums, origins of large inclusions is the entrapping large inclusions against casting speed is accelerated of slag in the converter and of slag in the tundish. wi th increase of the size of bi llet section, and the The fact that large inclusions are found in greater other negative in that the coagulation and growth of amount in the strand near the pouring stream from inclusions are a ccelerated in the mushy zone, where the ladle than in the strand far from the pouring stream solid and liquid pha es coexist, with lengthening of reveals that either the shorter travelling time of molten the period of solidification. steel in the tundish is unfavorable to the floating-up From the foregoing, the following will be effective of incl usions or disturbance of the pouring stream to decrease large inclusions: to prevent air or slag into the mould causes the amount of air oxidation to oxidation of the molten steel, to prevent the entrapping lI1 crease. of slag and scums, and to decrease oxygen before deoxidation or accelerate the Aoatating-up of inclu 2. Air Oxidation of Pouring Stream info Nlould and Sions. Trapping qf Scums REFERENCES In com pari on with open nozzle billets, billets with submerged nozzle and powder casting to prevent air I ) H. Mo ri el al.: T etsll-to-Hagani, 55 ( 1969). S 11 3. 2) F. Korbe r a nd W . Oelsen : M itt. K-W Insi. Eisen/orsc". , 15 invasion show lower total oxygen, higher sol. Al %, ( 1933),27 1. and smaller number of large inclusions. 3) Y. Nagano el 01. : T etsll-to-Hagani, 52 ( 1966),556. The effects of submerged nozzle and powder casting 4) J. H. Walsh and S. Ramanchandran : Trans. Nlet. Soc . are to preven t the air oxidation of the pouring stream, A IME , 227 ( 1963). to prevent the air entrained into molten steel in 5) J apan Society for the Promoti on of Science, Steelmaking the mould by the pouring stream, to prevent the air 19th Committee : Recommended Equi librium Data of oxidation at the bath surface, and to prevent the Steelma king R eacti on, ( 1968). entrapment of scums. The a ir oxidation theory is 6) S. Watana be et 01. : Tetsll-to-Hagani, 55 (1969). S 135. supported by the presence in AI-killed open nozzle 7) M . Tsunoi (Mitsubishi H eavy Industries, Ltd ., Hiros hima billets of Mn-silicate large inclusions which are not Shipyard): Private Communication. in equilibrium with the molten steel composition. 8) M . Gohler: Freiberge r Forshungsheff, B9 3 ( 1963). Adachi a nd Ogino: Report of J o int Iron a nd Steel Basic In open nozzle billets the number of large inclusion Research Socie ty, Molten Steel a nd Slag Di vision, ISIJ, increases with addition of powder in the mould, and ( 1967) p.91 large inclusions having a composition originating from 9) H . M ori et 01.: Yawala T echnical Report, No. 255 ( 1966). the powder are detected in the billets. The fact that 6537. the number of large inclusions in creases with decrease 10 ) Y. Nakagawa and S. Momose: TetJU-to- Ha.l!,ani, 53 ( 1967). of the melting temperature, the viscosity of scum in 397. Report