PREPARATION STUDY OF SEMI-SOLID 60Si2Mn SPRING STEEL SLURRY

ACTA METALLURGICA SINICA (ENGLISH L.EFTTERS)Vol. 16 No.6 pp 483- 488 December 2003PREPARATION STUDY OF SEMI-SOLID 60Si2Mn SPRINGSTEEL SLURRYW.M. Mao, A.M. Zhao, D. Yun, L.P. Zhang and X.Y. ZhongUniversity of Science & "Techiology Beijing, Beijing 100083, ChinaManuscript received 14 November 2002; in revised form 15 April 2003The nondendritic semi- solid slurry preparaton of 60Si2Mn spring steel is studied inthis paper. The erperiments showthat when stired for 2 minutes under the test con-dition, the semi-solid slurry with 50%- -60% fraction sold phase and spherical primaryarustenite in the size of 100-30qum can be obtaned and is easy to be discharged fromthe lttle bottom hole of the stirring charnber. The more homoyeneous temperaturefield and solute distribution of the 60Si2Mn spring steel mel appear because of theelectromagnetic stirring, whach restrains the formation of large primary austeniticdendrtes and create a base to form spheroidized structure. The stronger temperaturefluctuation in the melt with many rosette primary austenites caused by stirring andthe remelting of the secondary arm roots in the same time are the most imporlantreasons for deposition of spheroidized primary austentes.KEY WORDS 60Si2Mn, semi-solid, electromagnetic stirring1. IntroductionThe semi-solid forming technology of metal alloys was invented in the early 70s of the20th centuryl1l. This new technology has many advantagcs, such as reducing porosityand soldification shrinkage in the castings so that their compactness and strength are in-creased, reducing the solute segregation and improving the property's uniformity, reducingthe deformation resistance so that the sophisticated castings can be formed, raising theproductivity and reducing the percentage of rejects, lowering the forming temperature andprolonging the die life, and easily realizing the production automatization, so the semi-solid forming technologies have been used sucessfully in manufacturing aluminun alloyand magnesium alloy parts[2- 4While the semi-solid forming of alloys with low melting point was studied, the semi-solid forming of ferrous alloys had also been researched on a some scale. For example, .Flemings and his fllows of MTT prepared the semi-solid slurries or billets of AISI440C, 304,4340 steels with mechanical stirring equipments, did the semi-solid die casting experimentsand gained compact castingsl2,5-8]. Kapranos et al.9l also prepared semi-solid billets ofM2 high speed steel by Ospray or SIMA methods and carried o1t forming experiments.Some progress in continuous semi-solid cast billets of alloys with high melting point byelectromagnetic stirrer was achieved by Blazek et al.f10]. The main reason delaying thetechnical improvement of die casting process with semi-solid iron or steel slurry is thatthe cracks on the die surface usually occur due to thermal shock of high melting pointslurry resulting in shortness of the die life, so it is ought to develop continuously thesemi-solid forming technology suitable for high melting point iron or steel slurry. Thedirect rolling process of iron or steel slurry should be the better way because the roller中国煤化工MYHCNMHG484shape is simpler and the slurry is supplied continuously; and it is possible for the rollersto withstand the thermal shock of high rmelting point iron or stccl slurry. In semi-solidrolling of iron or stel, the mechanical stirring, SIMA. Ospray inethods are not properfor preparation of iron or steel slurry bcause the life of mechanical stirring chamber andstirrer is short and they may pollutc the slurry easily. and because the SIMA and Ospraymethod arc only used for preparation of semni- solid bilets. Therefore, the electromagneticstirring may be the important method for continuous production of the iron or steel slurry.Up to today, however, the rescarch results of the preparation of iron or steel shurry bythe electroragnetic stirring method and slurry direct rolling of iron and steel are rarelyseen in published papers. So in this paper it is studied that the preparation of 60Si2Mnspring stccl semi-solid slurry by clectromagnetic strring and the semi-solid slurry withsolid fraction of 50% 60% is sccessully prepared. which has provided the experience andexpcriment basis for stccl slurry rolling process.2. ExperimentalThe raw material in the experiments is 60Si2Mn spring steel with 0.61%C, 1.83%Si,0.7%Mn,≤0.05%P, <0.03%S,≤0.35%Cr, <0.35%Ni. The mclting furnace is intermediatefrequenicy coreless induction furnace and 20kg of the steel are melted every tine. Theproper amount of pig iron is added in the rmelt for compensation of the burning loss of Cand Si, the stccl melt is deoxidized by Si-Ca powder and heated to 1600°C before tappingin order to rneet the requirernent of pouring and heating the stirring chamber liner. Aftertapping, the liquid steel is poured into the stirring chamber of the semi-solid steel slurrypreparation equipment and stirred immediately by electromagnotic fieldl.The equipment for preparationand rolling of nondendritic semi-solidsteel slurry by electromagnetic stir-2ring is shown in Fig.l. The stirringchamber is made from sintered sili-con carbide tube and the center blockbar is made from tow coaxial aluminatubes, which enables the rotary clec-tromagnetic field to penetrate into thestirring charmber center as much aspossible. As the superheated liquid60Si2Mn spring steel is poured intothe stirring chamber, it is stirred rightaway by electromagnetic field andcooled continuously. When the semi-,一dosolid melt is stirred, the cooling rateis appropriately controlled so that thesemi-solid melt can stay between theFig.1 Sethuei ic of preparativnn Hudl rlling ul theliquidus temperature Ti and solidussemi-solid steel slurry.1. center block bar, 2. pouring basin, 3.temperature Ts for enough long timestirring chamber, 4. cloctronagnetic stir-and the slurry with spherical or nearlyrer. 5. heating element, 6. slurry, 7. cool-spherical primnary austenitic grainsing water. 8. roller. 9. rolled plate中国煤化工MYHCNMHG485.can be easily gained. When the slurry reaches the given solid fraction, the center blockbar of the equipment can be risen up, and the scimisolid slurry flows out of the stiringchamber from the lttle bottom hole and then goes on fowing into the predeterminedgroove between the two hollow rollers and finally is rolled into plate.In order to roll semi-solid steel slurry, the process parameters should be determinedfirst by experiment. The temperatures in the chamber top and bottom are determinedby PtRh-Pt thermal-couples and the chamber is prehcated to 1200- 1350°C before pouringliquid 60Si2Mn spring steel. The center block bar is risen up after the steel melt is stirredfor a series of given time, and a small of arnount of semi-solid stccl slurry flows out of thechamber and is quenched into a water pool, and the center block bar is again lowered tothe origin place and closes the bottom hole. The stirred and quenched 60Si2Mn springsteel is made into metallographic samples. The samples are roughly and exactly polished,and then etched by aqueous solution of picric acid, and observod with the help of opticalmicroscope so as to analyze the shape and distribution of the spherical primary austeniteand decide the preparation process pararncters of semi-solid 60Si2Mn spring steel slurry inthe end.The experiment conditions of semi- solid 60S5i2Mn spring stcel by electromagnetic stir-ring are shown in Table 1.Table 1 Experiment conditions of semi-solid 60Si2Mn spring steel by ele:tro-magnetic stirringSample No. Preheat temperature of Stirring time Stirring power Heat numberthe stirring chamber, °CkW135030.8)00.81201200).823603. Results and DiscussionsWhen the stirring chamber is preheated to 1350°C, the liquid steel with the tappingtermperature of 1600°C is poured into the stirring chamber and stirred for 60 seconds under9.8kW power. Then, about 30vol.% of primary austenitic grains appear in the steel melt,and the austenitic grains are easily distinguished from the quenched liquid steel becausethe stirred nondendritic primary austenitic grains are larger than the quenched liquid steel.The average size of the stirred primary austenitic grains is about 100- 300μm at the earlystirring stage, and most of them are rosette and small parts of them are spherical ornearly spherical, as shown in Fig2a. This slurry is easy to flow from the little bottomhole of the chamber because its solid fraction is lower. The solid fraction of the slurrycontinues increasing and the spherical austenitic grains are more and more w hen stirredfor 90 seconds, but there are also some rosettc austenitic grains in the serni-solid mmelt, asshown in Fig.2b. As the serni-solid melt is stirred for 120 seconds, it may be scen that theprimary rosette austenitic grains almost disappear and the spherical austenitic grains arerounder, some spherical austenitic grains, however, are connected together again, as shown中国煤化工MYHCNMHG， 486in Fig 2c, and that the solid fraction of the slurry is about 50vol.% -60vol.% at this timeand it is to fow ceasily from the little bottom hole of the chamber too, but sorne slurry maybe stuck on the inner top surface in the chamber because the chamber top is cooled morequickly. The most aunount of the semi- solid slurry discharged is 15kg in these experimentsunder the above experiment conditions and more than 10kg of the slurry can be dischargedfrom the chaumber every time. If stirred for longer time. the solid fraction of the slurry istoo high and it is ificult to flow out from the lttle bottom hole. The stirring chamberwas reheraterd to 13509C beforr poring for samplos (n c) and 12009C for sanple (d).200um100m，200umFig.2 Micrustructures of semni-solid 6USi2Nn spring steel stirred by electro-magnetic field for 60 seconds (a). 90 seconds (b), 120 seconds (c) and120 seconds (d).When the stirring chamber are preheated to 1200°C, the liquid steel with the tappingtemperature of 1600°C is poured into the stirring chamber and stirred for 120 secondsunder 9.8kW power. Then, more than 60voL.% of primary austenites appear in the steelmnelt and most of the primary austenitic grairns are spherical, and the averagc size of thestirred primary austenitic grains is 100- -300μm, as shown in Fig.2d. But, it is discoveredthat this slurry is more dificult to flow from the chamber and much of the semi-solid slurryis stuck on the inner top surface of the charmber so that only a fcw kilograms of semi-solidsburry of 60Si2Mn spring steel are discharged every time.中国煤化工MYHCNMHGThe primary austenitic grains of 60Si2Mn spring steel will solidify to dendritic grainson the traditional condition. From the above experiments, however, the primary austenititegrains of 60Si2Mn spring steel will become spherical under the electromagnetic stiring con-dition, which may be related with the ollowing reasons: First, the temperature and solutedistribution of the melt stirred by electrorlagnetic ficld are made more homogeneousltl,the whole melt of 60Si2Mn spring steel is almost cooled to the same temperature, meal-while the whole area of the melt almost nucleates everywhere so that the preferred growthof the austenitic dendrites is confined and fine primary rosette austenitic grains grow fromthe melt, which are shown with arrows in Figs. 2a and 3a. Therefore a base is created forgetting spherical primary austenite grains. Second, the authors' former experiments showthat the semi-solid melt stirred by electromagnetic field is mainly turned horizontally, andmeanwhile an additional flow existed, by which the melt frequently moved up along theinternal wall of the chamber and then down to the center. Consequently, these fine primaryrosette austenitic grains with the liquid corne to the warmer center area one moment andgo to the cooler peripheral area of the stirring chamber the next, and the violent temper-ature fuctuation is induced by this fow motion and accelerates the remelting of a largeamount of the secondary arms on their roots, and many spherical or near spherical primaryaustenite grains will arise. The necked secondary arms are shown with arrows in Fig.3band these necked secondary arms will gradually remelted on their roots and scparated fromthe primary austenite in the later stirring. Third, the stirring time is an important factorleading to the formation of the spherical primary austenitic grains. If the stirring time islong enough, all the secondary arms of the finc primary rosette austenite will be separatedaway finally. The microstructures stirred for longer time support this viewpoint. Fourth,the stirring power is another important factors giving rise to the formation of the sphericalproeutectic austenitic grains. The larger the stirring power is, the more vigorous the slurrymotion will be, and the stronger the temperaturc fuctuation of the fine proeutectic austen-ite will be, and it is much more possible for the sccondary arrns to remelt on their roots.Because the stirring powcr. which is dctermined in other experiments, in this experiments6F50um100/mFig,.3 .Microstructuros of semi-solid 60Si2Mn spring steel stirred for 90s underelectromagnetic stirring power 9.8kW and the reheating temperatureof chamber 1350°C: (a) finc primary rosctte Hustenitic grains appear;(b) necked secondary arms apear.中国煤化工MYHCNMHG488is enough large, it is possible to obtain required slurry of 60Si2Mn spring steel so long asstirring for proper time. In addition, the primary austenitic particles will become moresphericeal becase the primary austenitic particles ollide and rub strongly with each otheror with the liquid.4. Conclusions(1) When the strring chamber is preheated to 13509C: the tapping temperature ofthe liquid 60Si2Mn spring steel is 1600°C, and the strring time is 2 minutes on the givenpower condition, the proper semi-solid slurry with 50vol.% 60vol.% primary solid austeniticgrains can be prepared. The size of the grains is about 100- 300μm and the slurry is easilydischarged from the chamber.(2) The more homogeneous temperature and solute distribution of the 60Si2Mn springsteel mclt appear because of the electromagnctic stirring; which restrains the formation oflarge primary austenitic dendrites and creates a base to form spherical crystals of primaryaustenite. The stronger temperature fluctuation in the melt with many primary rosettealusternitic grains and the remelting of the secondary arm roots in the same time are themost important reason for deposition of spherical primary austenitic grains.Acknouledgements-- This work was supported by the Nutional Natural Science Foundation of China(No. 59995440).REFERENCES1 D.B. Spencer, R. Mehrabian and M.C. Flemings, Metall. Trans. 3A (1972) 1925.2 M.C. Flemings. Metalurgrcal Transactions 22A(5) (1991) 957.3 M. Garat and S. Blais, in The 5th Inl. Conf. on Sermi Solid Processing of Alloys and Comnposites, edsA.K. Bhasin, JJ. Moore, K.P. Young and S. Midson (The Colorado School of Mines, Colorado, Golden,USA, June 23-25, 1998) p.199.4 P. Eisen and K. Young, in The 6th Int. Conf. on Serei-Solid Processing of Alloys and Composites,eds G.L. Chiarmetta and M. Rosso (The Department of Materials Science and Chemical Engineering,Politecnico Di Torino, Turin. Italy, Sept. 27-29, 2000) p.41.5 J.M. Oblak and W.H. Rand, Metallurgical Transactions B 7(12) (1976) 699.6 J.M. Oblak and W .H. Rand, Metallurgical Transactions B 7(12) (1976) 705.7 D.A.V. Cleave, lron Age 220(8) (1977) 34.8 K.P. Young, R.G. Riek and M.C. Flemings, Metals Technology 6(4) (1979) 130.9 P. Kapranos, D.H. Kirkwood and C.M. Sellars, in The 4th Int. Conf. on Sermi Solid Processing ofAlloys and Cormposilions, eds. D.H. Kirkwood and P. K apranos (The Universiy of Sheffield, England,UK, June 19-21, 1996) p.306.10 K.E. Blazek, JE. Kelly and N.S. Pottore, ISIJ Intermalional 35(6) (1995) 813.11 W.M. Mao, A.M. Zhao, C.L. Cui, F. Sun, C.L. Zheng and X.Y. Zhong. Acta Metallurgica Sinica 36(5)(200) 539 (in Chinese).中国煤化工MYHCNMHG