Murakami, Katuhiko

Suzuki, Mikio

Yamaguchi, R_yuji

Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura

IVoLI3(I)(1995) (Proceedings ofthe Intemational Conference "Processes and Maîerials Innovation in Stainless Steel products,,Florence, haly, I 1-14 October 1993").Conlrol of Uneven Shell Fornqfion of Stqinless inEcrly Stcges of SolidificqtionR_yuji Yamaguchi, Mikio Suzuki, and Katuhiko MurakamiMaterials & processing Research Center t{KtiCoiporation, Fukuyama, JapanAbstractThe surface cracks thaî are originated by an uneven solidified shell formation in mold tend to occur frequently in continuously cast slab of austeniticsîainless steels' "soft" cooling mold is one of advantageous methods îo decrease these defects, hà*"in, there is not enough.An investigation hasbeen carríed ouî into the mechanism of unevei solidífiei shelt formation at the initial sa^ge_of sotidification by dipping oJ water cooled chíll plates inthe molten stainless steel' st1.e! srlde.eytntoyedfor ihe drp tests is AISI304, 316 and 430. The chill'plaîes naí" u"Ln "*ptoyedwith Ni plating on theentire Cu sutface and with Ni laniced plating lZ,tO,tSmm pftcnl.The experimenîal results indicated rhat, 1 t 1 in" sn"u i7 iustenitic stainless steer has a irregurar roughness and these roughness form aír gap betweenthe shell and the chilt plaîe and results in the un"vei iiiai6"a^rnat 7or*iit".'izl *, chíll plate with Ni latticed prating (tomm pìtch) was mosîeffective to prevent the uneven solidified shell formation. This effect caí o, "-iuiìli i"'*rms of the dispersion of coitractionJ.orce into the sotidifiedshell' Nextly' the casting tests usins'a small cbntinuous casting machine witi this *ord "ontroir"d rrrr^ h"ot ,"tio.rtio, *"r" p"ryormed to investi-gate îhe measures for prevention oj the uneven solidified s;hell"formation.RiassuntoLe cricche superficiali generate dalla formazione irregolare del guscio solidificato frequentemente si manifestano nelle lastre d,acciaio austeniticoottenute per colata continua.L'uttlizzo di uno stampo "tenero" per il raffreddamento porta ad una riduzione dell'inciden za di talidifetti, ma non alla eliminazione totale degli stes_si' Questo lavoro esamina I'effetto sul meccanismo dÉlla formazione i.r.;o;; a"i'gur.io solidificato all,inizio dela solidificazione provocatodall'immersione nell'acciaio inossidabile fuso (AISI 304,316 e430) di due ípi ài ru*Jài rame raffreddata ad acqua: una compretamente rivesrita diNi; I'altra rivestita a reticolo (passo 2, 10 o 15 mm). I risultati hanno dimostràto "he: ft; l" ,ogosità, irregolari che vengono a fbrmarsi sulla superficiedel guscio dell'acciaio in oggetto creano uno spazio riempito d'aria tra il guscio ,,".* " tu lastra che pàrta aila roÀu?ion" irregolare del guscio allostato solidificato; (2) tra le lastre sperimentate, quella rivestita a reticoloipasso to Àm; e.u la più efficace nel prevenire tale formazione. Si ritieneche questo effetto sia dovuto alla dìspersione delle forze di contrazione aent o it guscio solidificato. Sono state àoltre eseguite delle prove di colatacontinua su una piccola colaÎrice con I'impiego del metodo descritto per il controlìo deila estrazione non uniforme nello stampo a'o scopo di esami-nare gli interventi necessari per prevenire la fórmazione irregolare deiguscio solidifiau,o-" """'"'rv 'u, ''u.'t rKey Words:continuous casting, Stainress steel, surface crack, uneven soridification, Depression, Dip testslntroductionDuring continuous casting ofhypo-peritectic carbon steel and austenitic stainless steels, surface defects suchas surface cracks2) and dépressiàn3) often occur from uneven sotioffication ìt;; ,h";old. Many studies ofuneven solidification phenomena that accompany this kind of peritectic reaction in steels have been repor-ted' Grill et al'4) and Sugitani et al.5) maintained that contractioì of the solidifying shell during ò/y transfor-mation recurves the shell and then floats it from the mold surface, causing an uneven transfer of heat thatresults in uneven growth of the shell. On the other hand, many reseÍpchers have also worked on methods forreducing surface defects such as surface cracks. sugitani "i"rft*p"rr.Jìi", *r,,, cooling of molds iseffective for reducing the contraction volume gap between the mold and molten steel sides of the shellduring the early stages of solidification and also ror reducing the shell strength. consequently, bending ofthe shell toward the molten steel can be prevented, allowing tie shell to grow without separating itself fromthe mold' Therefore, approaches have beèn attempted to prwent surface cracks, such as reducing the volumeof mold cooling waterT) a-nd lattice grooving on rhe mold surfac;ttti io-.dl;'"bp l.lottn, plate capable ofcontrolling uneven solidification during the casting of stainless steel, the influeìce of the cooling plate onuneven solidification was examined in this study using a dip test of cooling plates into molten steel and acasting test using a small continuous casting (CC) machine.Method of Dip TestDip Test EquipmentTo examine the properties of the solidifying shell, an internally water-cooled copper plate was dipped intomolten steel. Fig. 1 shows the test equipment, which consists of an air cylinder for moving the dip piece upand down, a platform and the dip piece. The dip piece consists of a watercooled copper plate, a coolingjacket and castable sections. All face plates of the cooling jacket except the water-cooled plate are coveredwith Al2O3 castable so that the cooling jacket does not contact the molten steel directly.After preparing the molten steel to the specified composition and temperature in a IOO kg induction furnace,the dip piece was immersed into molten steel using the air cylinder.The piece was then lifted up immediately after the given time period to permit examination of solidifiedshell formed on the water-cooled plate surface.Dip Test GonditionThe condition for the dip test are listed in Table 1. Three steel types were used, i.e., AISI 304 and AISI 316,which are typical austenitic stainless steels, and AISI 430, which is a typical ferritic stainless steel. The mol-ten steel temperature during the dip test was kept constant at the liquidus temperature of each steel +50' C.A l0 mm thick Cu plate was used as the cooling plate material for the dip pieces, and approximately 90 Uminof water was used for cooling.To confirm the "soft" cooling effect of the cooling plates, the cooling plate surfaces were manufactured intwo ways. First, the entire surface of the cooling plates was Ni-plated in two thickness of 0.2 mm andO.5 mm. As an alternative, the cooling plate surfaces shown in Fig. 2 were Ni plated to form a lattice. Threedifferent cooling plates were used, and the surfaces were Ni plated to have lmm thick and Smm wide stripsat intervals of 2mm,10mm and l5mm.Three dip periods of 5 sec, lO sec and 15 sec were used, and the dip speed was set at approximately400 mm/sec. The method shown in Fig. 3 was used to evaluate the unevenness of solidified shells formed onthe cooling plates. The top and bottom ends of the shells formed on the cooling plate surface were croppedand the remaining approximately 200mm was cut longitudinally to measure the thickness of the solidifiedshells at intervals of 5mm.The unevenness parameter of the solidified shells (oD() was derived from X, the average thickness, and o,the standard deviation.MoltenSteelWatercooledplateSUS3O4, SUS316, SUS43OAT=50+5oCCu10 mm90 VminNi plating on the over-all - Cu surface (Ni thickness 0,0.2,0.5 mm)Ni latticod plating on the Cu surface(lattice spacing 2,10,15, mm)TABLE î - Experimental condition for dip testDipping time 5, 10, 15 sec.Metallurgical Science and TechnologyTest Results and GonsiderationsDip.Test ResultsPhoto. 1 shows the influence of the Ni plated thickness on the unevenness of solidified shells forrned on thecooling plate surface. For the austenitic AISI304 and AISI316 steels, the shells that solidified on the originalunplated Cu plates are characterized by a high level of unevenness, as shown in this Photo. However, theunevenness decreases with increasing Ni plating thickness. These solidified shells are concave on the coo-ling side, as opposed to also being concave on the molten steel side. This suggests that in the early stages ofsolidification, the solidifying shell is concave on the molten steel side as s result of a thermal shrinkage gap.This gap is caused by an uneven temperature distribution beneath the shell surface, which in turn suppressesheat transfer to the cooling plates, delaying subsequent solidification.In the case of ferritic AISI430, on the other hand, evenly solidified shells were formed under all conditions,suggesting that the smaller the peritectic reaction in stainless steels, the lower the probability of uneven soli-dification, as is the case with carbon steel.Fig. 4 summarizes the influence of the Ni plated thickness on unevenness (o/X) of the solidified shells. Forthe Cu plate that was not Ni plated, unevenness of the solidifying shell increased with the dip time. On theother hand, the unevenness parameter decreased with increasing Ni plating thickness. In particular, for thesample Ni plated to a thickness of 0.5mm, the unevenness parameter of the solidified shell decreased withincreasing dip time.Fig. 5 shows the differences due to different steel types. As is seen from Photo.1, AISI316 was the most une-ven and AISI430 the most uniform. It is also shown that regardless of steel type, with a Ni plated thicknessof 0.5mm, uniformity improved as the dip time was extended.Photo. 2 shows cross sections of the solidified shells formed on plates that were Ni plated to form a lattice.Solidified shells formed on Ni plated lattices at intervals of 2mm or 1Omm featured sufficient evenness andfew thickness fluctuations, as with the cooling plates Ni plated to a thickness of 0.5mm over the entire surfa-ce. When the pitch was widened to l5mm, however, the unevenness increased.Fig. 6 shows the influence of lattice pitch on the unevenness of solidified shells for AISI316, which had thegreatest unevenness of the samples that were entirely Ni plated. Under the conditions of this test, 10mm wasthe most effective lattice pitch capable of improving unevenness.It was shown that extension of the dip time did not affect unevenness. On the other hand, Ni plating at inter-vals of 15mm produced similar results in the initial stage of dipping to that of the 2mm pitch, but uneven-ness increased as the dip time was extended, ultimately producing a result similar to that with the surfaceentirely Ni plated to a thickness of 0.5mm.Heat Transfer BehaviorThe method of assessing uneven solidification from the shape of the solidified shell was mentioned above.Change in the thermal transfer coefficient between the cooling plate and solidified shell that shows the con-ditions of contact between the cooling plate and solidified shell is important. Accordingly, the influence ofNi plating on the thermal transfer coefficient between the cooling plate and solidified shell was studied usingthe one-dimensional,transient-state thermal transfer calculation model shown in Fig. 7. The thermal transfercoefficient on the cooling water side, hw, was derived from the formula shown in the Figure by using TA,the measured temperature of the Cu plate interior at the point of time when heat flux to the cooling waterwas nearly stabilized. The term hs is the thermal transfer coefficient between cooling plate and solidifiedshell.The hs at which the calculated temperature and the measured temperature, TA, coincided was determined byvarying hs within a set range. Fig. 8 shows how this hs changed from the start of dipping. When an unplatedCu plate alone was used, the thermal transfer coefficient was obseled to fall sharply within a very shorlVol. t3(t)(t99s)time after the start of dipping. This suggests the development of thermal resistance such as an air gapbetween the cooling plate and solidified shell. On the other hand, the thicker the Ni plating, the larger thethermal transfer coefficient becomes. This is the same tendency as the influence of Ni plating thickness onthe unevenness of the solidified shell as seen from its shape. What deserves special attention is that the ther-mal transfer coefficient increased for about 2 seconds after immersion for both the AISI304 sample Ni pla-ted to a thickness of 0.5mm and the AISI430 sample plated to 0.2mm. This suggests that for the first twoseconds of solidification, the conditions of contact between the solidified shell and the cooling plate impro-ve, eliminating unevenness of the solidified shell. The interaction between the rate of heat transfer from thecooling plate, the corresponding thickness of the solified shell and the hydrostatic pressure of the moltensteel are considered to be determining factors although the details remain for future investigations.Casting Test with Small GG MachineUnlike actual casting, dip test results were obtained under conditions in which mold powder was not a factor.It was anticipated that if this method was applied to actual casting, the effect of the lattice-formed Ni platingwould be offset by the effect of "soft" cooling by the mold powder.Therefore, reconfirmation was made by using a small CC testing machine to provide conditions similar tothat of an actual casting machine.Test MethodFig. 9 shows the test equipment and method of mold manufacturing. This is a small CC machine capable ofcasting a 300kg slab that is 310mm wide and 80mm thick. The test casting was made after a wide face ofmold was Ni plated to make a lOmm pitch lattice. Based on the dip test results, this configuration was thou-ght to improve uneven shell solidification. As shown in the Figure, the lattice Ni plating extended down300mm from the mold top. Table 2 shows the casting conditions. A low viscosity / melting point powderwas used for casting. Fe-S was added during casting to define the shell in the casting process. Fig. 10 showsthe method of measuring the unevenness parameter of the solidified shell. The solidified shell was locatedby using sulfur prints at 1/4 of the billet width in the longitudinal section of the casting direction.The solidified shell thickness was measured at intervals of 2mm. An approximate curve was determinedfrom the measured thickness of the solidified shell by using the rule of root 1d=A+Br/t). The deviation of theapproximate cunre and solidified shell thickness (area S ) was defined as the unevenness parameter.TABLE 2 - Casting conditions for small sized test casterMolten steel 300 kgMachine typeCasting speed 0,6 ,0,45 m/minOscillation+ 5 mm 90,67 cpmNon sinusoidal (o = 207o)Steel grade AISI3O4AT in ladle 45+5KM etaL Iu r g ical S c ie nce and Te chnolo gyTest ResultsResults obtained under various conditions are summaized in Fig. 11. The unevenness is reduced by usinglatticed Ni plating under all conditions. Based on the shell's coefficient of solidification, the overall growthof the solidified shell is similar between the cases, as Fig. 12 shows. The actual casting tests using powderalso confirmed that the Ni plated lattice mold could improve uneven shell solidification.ConclusionDip tests were performed to study uneven shell solidification, which is a problem for casting stainless andother steels. These tests provided a method for forcibly and unevenly cooling the surface on which the soli-dified shell is formed. It was found that uneven solidification could be improved by the use of lattice-formedNi plating at an optimum spacing. A casting test using a small CC machine subsequently confirmed thatuneven solidification could be improved even if powder was used. A future study is scheduled to confirmthis effect by using an actual CC machine and to clarify the relationship between uneven solidification andslab surface defects.References[1] S.N.Singh and K.E.Blazek: Journal of Metals,26 (I974),I0,p.17[2] N.Miyasaka: Tetsu-to-Hagane, 64 (1978), s663[3] M.Wolf: Iron & Steelmaking ,13 (1986),p.248[4] A.Grill and J.K.Brumacobe: Ironmaking Steelmaking,2 (I976),p.76[5] Y.Sugitani: Tetsu-to-Hagane, 65 (197 9), p.17 02[6] Y.Sugitani: Tbtsu-to-Hagane, ó7 (1981), p.508[7] J.Nagai : Tetsu-to-Hagane, 69 (1983), s.158[8] K.Nakai : Tetsu-to-Hagane, 73 (1987), p.498Vol. 13 (1) (1995)Air ] rkliliFio î'The method of Ni latticed platingcylinder'Water cooleplateAlzOzcastableooooo,, Inductionjl'z' fumaceo-rFig. 1:Schematic view of experimental apparatus for dip testFig.3:Evaluation method of unevenness parameter of shell (o/X)| , o î | ^sh.il2li=|" î0.4l, o,r*- Bottomr Î02ll';Unevennessparameter(oDQ O 00510Is20o: SlandarddeviationolsheUthickness v J rv rJ zv)lAverageofshellthickness Dipping time (sec)Fig.4:Influence of Ni thickness on the unevennesparameter of shellMetallur g ic al S c ienc e and Tec hnolo gyPhoto. 1:Influence of Ni thickness and steel grade on shell formationSUS3O4 SUS316 SUS43ONi latticedplating2mmpich-b' iit[fli| '' .tm.trrF INilatticedplatinglOmrnpict--idi t r r.t,rFFL, l..t.i.l r, rNilatticedplatingl5mmpich---. r^l*l^r I l.tm.rJ*r.!Ll-lsji I l. I i I ,UJl"li , I i : i :,: irmPhoto.2:Effect of Ni latticed plating on shell formationVol.13 (1)(1995)^A0.5mm Ni-Tlsm4-AAISI3l6r AISI43O! 0.-5Éaî04I0.351015Dipping time (sec)Fig.5:Influence of steel grade on theunevennes parameter of shell.Fig.7:Calculation model of heat transfer at non-steadystate to estimate the thermal transfer coefficientbetween cooling plate and shell.! b - ----=-; L-o o-s^*JlE .rF,, ",,,,,,,,,,,,,,,,,-,ll:l"0510152025i Dipping time (sec)0.20.10.3Xb, 0.2IIY o.rFig.6:Influence of lattice spacing onunevennes parameter of shell::l1JLhÒIOl!Oo()oL.vlgtÉoE8o!&LFcd6)ÈiÌilFig. 8:The change of thermal transfer coefficientbetween cooling plate and shell.f ArsBrollr 2mm Ilrtomm Ilrts'n* II o jm- I0o2 M.lt nATQwW:Cw)t:T,ITn:TwL:hwTemoerature differenceof criolingHeat fluxFlow rate of waterSpecific heat of waterDensity of waterThemal conductvity of CuMeasured temDeratureinside Cu pÌatéTemoerature of Cu surfaceon cóoling sideTemperature of cooling waterLrcation of TA from cooling sideThermal transfer coeffi cientat water sideQw = Cw . 9w .W. AT/STs=Te-Qw.UÀ}1y = Qw/ (Ts-Tw)hs : Themal transfer coefficientat molten steel sideS : Area ofcooling plateMetallur gical Scienc e and Technolo gyFig.9:Schematic of small-sized test caster and Nilatticed plating method on mold surface.Sulphur printFig. 10:Evaluation method of unevennes parametermeasured from sulphur print.Cu Plate Ni latticedplatingFig.12:Effect of Ni latticed plating of thesolidification constant.nl lq10Cu plate\ffi_,rNi latticedplatingc.lra)llq()J4ov)e320oOdaU)-tr50 IdEEaa!(.)(J$ rooo.oooNi latticedplatingFig. 11:Effect of Ni latticed plating of unevennessparameter of shell.11Ni latticed plating sideNi latticedplatingCu plate4(nd=A+BVtDistance from meniscusd=A+BVtVcm/minfcomC 0.63A o.+str 0.61909067Vot. 13 (1) (199s)t0

Conlrol of Uneven Shell Fornqfion of Stqinless in Ecrly Stcges of Solidificqtion

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Conlrol of Uneven Shell Fornqfion of Stqinless in Ecrly Stcges of Solidificqtion

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