Slurry Preparation and Rolling of Semi-solid 60Si2Mn Spring Steel

J. Mater. Sci. Technol, Vo1.19 No.6, 2003613Slurry Preparation and Rolling of Semi-solid 60Si2Mn Spring SteelWeimin MAOt, Aimin ZHAO, Dong YUN, Leping ZHANG and Yonglin KANGSchool of Materials Science and Engineering, University of Science & Technology Beijing, Beijing 100083, China[ Manuscript received October 31, 2002, in revised form January 8, 2003]The nondendritic semi-solid slurry preparation of 60Si2Mn spring steel has been studied in this paper. The experimentshave shown that when stirred for 2 min on the test condition, the semi-solid slurry with 50%~60% fraction solid andspherical primary austenitic grains in the size of 100~300 um can be obtained and is easy to be discharged from thebottom lttle hole of the strring chamber. The nondendritic slurry of 60Si2Mn spring steel can be rolled into givenplate form scessfully, but the solid phase and liquid phase is easy to be separated in rlling process so that thesolid primary austenite is concentrated in the center and the liquid is near the edge of the rolled plate. The rupturestrength and elongation of the plate rolled only once with semi-solid slurry are lower than that of the traditionallyrepeated hot-rolled plate of 60Si2Mn spring steel.KEY WORDS: 60Si2Mn, Semi-solid, Electromagnetic strring, Rolling1. Introductionhas studied the preparation of 60Si2Mn spring steel semi-solidslurry by electromagnetic stirring and the slurry rolling. TheThe semi-solid forming technology of metals or their asemi-solid shurry with solid fraction of 50%~60% was preparedloys was invented in the earlyDs of the 20th centuryl1.and rolled into plate, which has provided the experience andThis new technology has many advantages, such as reducingexperiment basis for steel slurry rolling process.porosity and solidification shrinkage in the formed castings sothat their compactness and strength is increased, reducing the2. Experimentalcomposition segregation and improving the property unifor-mity, reducing the deformation resistance so that the sophistj-The raw material in the experiments is 60Si2Mn springcated castings can be formed, raising the production ratio andsteel with the composition of 0.61 wt pct C, 1.83 wt pct Si,reducing the percentage of rejects, lowering the forming tem-0.7 wt pct Mn, <0.05 wt pct P,≤0.03 wt pct S, <0.35 wtperature and prolonging the die life, and easily realizing thepct Cr,≤0.35 wt pct Ni. The melting furnace is intermediateproduction automatization, so the semi-solid forming technol-frequency coreless induction furnace and with a capacity ofogy of aluminum alloys and magnesium alloys has been suC~-20 kg of the steel. The proper amount of pig iron was addedcessfully used tomanufacture many parts of cars and otherin the melt for compensation of the burning loss of C and Si,the steel melt was deoxidized by Si-Ca powder before tappingWhile the semi-solid forming of alloys with low melt-ing point was studied, the semi-solid forming of ferrous a小pouring and heating the stirring chamber liner. After tap-loys had also been researched, for example, Flemings and hisping, the liquid steel is poured into the stirring chamber ofcolleaguesl2,5~5l of MIT prepared the semi-golid slurries orthe semi-solid steel slurry preparation equipment and stirredbillets of AISI440C, 304, 4340 steels with mechanical stir-immnediately by electromagnetic field.ring equipments, did the semi- solid die casting experimentsThe equipment for preparation of nondendritic semi solidand gained compact castings. Kapranos, et al.l9, also pro-iron or steel slurry by electromagnetic stirring and slurryduced the serni-solid billet of M2 high speed steel by Os-rolling is shown in Fig.1. As the superheat liquid 60Si2Mnprey or SIMA methods and carried out the forming exper-spring steel was poured into the stirring chamber, it wasiments. Some progress in continuous semi-solid cast billet ofstirred rightaway by electromagnetic fend wasoledalloys with high melting point by electromagnetic stirrer wascontinuously. When the semi-solid melt was stirred, the cool-achieved by Blazck, et al,1o. But, the main reason delay-ing rate was appropriately controlled so that the semi-soliding the technical improvement of the die casting process withmelt can stay between the liquidus temperature T and solidussemi-solid iron or steel slurry is that the cracks on the die sur-temperature T: for enough long time and the slurry of spheri-face usually occur for the thermal sbock of high melting pointcal or nearly spherical primary austenite can be easily gained.slurry and the die life is too short, so it is necessary to con-When the slurry reached the given solid fraction, the centertinuously develop the semi-solid forming technology suitableblock bar of the equipment is risen up and the semi-solidfor high melting point iron or steel slurry. The direct rollingslurry flowed out of the stirring chamber from the little bot-procesof iron or steel slurry should be the most importanttom hole, and then the semi-solid slurry went on flowing intoarea because the roller shape is simpler and the slurry is sup-the predetermined groove between the two holow rollers withplied continuously, and it is possible for the roller to withstandcooling water and finally was rolled into plate.the thermal shock of high melting point iron or steel slurry.In order to roll semi solid steel slurry, the process param-In order to suit the need of semi-solid iron or stel rolling,eters should be firstly determined by experiments. The tem-tbe mechanical stirring, SIMA, and Osprey methods are notperatures of the chamber top and bottom were determined byproper for preparation of iron or steel slurry because the me-PtRh-Pt thermocouples and were preheated to 1200~1350°Cchanical stirring chamber and stirrer life are short and theybefore pouring liquid 60Si2Mn spring steel. The center blockmay easily pollute the slurry, and the SIMA, Osprey methodsbar was risen up as the steel melt was stirred for a series ofare only used for preparation of semi-solid billets. Therefore,given time, and a small of amount of semi-solid steel slurrythe electromagnetic stirring may be the important method forflew out of the chamber and was quenched into a water pool,continuously producingthe iro or steel surry. Up to today,od the center hlock bar wAs arain lowered to the originalhowever, the research about the production of iron or steelplace; stirred and quenchedslurry by electromagnetic stirring method and their slurry di-60Si2中国煤化工stirnede andghlyrect rolling is rather few in published literatures. So this paperand fC N M H Goy aqueous solution ofpicric' acid, alu observeu usug upucal microscope so as to↑Prof, Ph.D., to whom correspondence should be addressed,analyze the shape and distribution of tbe spherical primaryE-mail: weiminmao@263.net.austenite and decide the preparation process parameters614J. Mater. Sci. Technol, Vol.19 No.6, 20033. Results and Discussion3.1 Preparation of semi-solid slurryWhen the stirring chamber are preheated to 1350°C, theliquid steel with the tapping temperature of 1600°C is pouredinto the stirring chamber and stirred for 60 s under 9.8 kWpower. Then, about 30 vol. pct of primary austenitic grainsappear in the steel melt, and the size and shape of theausteeniticgrains areclear from the quenched liquid steel,because the stirred nondendritic primary austenitic grainsare larger than those in the quenched liquid steel. The av-erage size of the stirred primary austenitic grains is about100~300 μm at the early stirring stage, and most of them arerosette and small part of them are spherical or nearly spher-ical, as shown in Fig.3(a). This slurry is easy to flow fromthe chamber little hole under the bottom because its solidfraction is lower. The solid fraction of the slurry continuesincreasing and the spherical austenitic grains are more andmore when stirred for 90 s, but there are also some rose-likeaustenitic grains in the semit solid melt, as shown in Fig.3(b). .Fig.1 Schematic of the semi- solid steel slurry roling,As the semi-solid melt is stirred for 120 s, it may be seen1-center block bar, 2-pouring basin, 3-chamber,that the primary rosette austenitic grains almost disappear4-electromagnetic stirrer, 5-heating element, 6-slurry,and the spherical austenitic grains are rounder, some spheri-7-cooling water, 8-roller, 9-rolled platecal austenitic grains, however, are connected together again,as shown in Fig.3(c), and that the solid fraction of the slurry isabout 50~60 vol. pct at this time and is easy to flow from thechamber too, but some slurry may be stuck on the chamberinner surface. The experiments show that more than 10 kg of10__250the slurry can be discharged from the chamber every time andthe most amount of the semi-solid slury discharged is 15 kgin thre experiments onabove experimentt conditions.When the strring chamber are preheated to 1200°C, theinto the stirring chamber and stirred for 120 s under 9.8 kWpower. Then, more than 60 vol. pct of primary austeniteappears in the steel melt and most of the primary austeniticFig.2 Shape and size of the samplesgrains are spherical, and the average size of the stirred pri-mary austenitic grains is 100~300 pm large, as shown itFig.3(d). But, it is discovered that this slurry is more dif-of semi-solid 60Si2Mn spring steel slurry in the end.ficult to flow from the chamber and mucb of the semi-solidThe semi- solid 60Si2Mn spring steel slurry was preparedslury is stuck on the inner gurface 。>f the chamber so thataccording to the above determined process parameters andonly a few kilograms of semi-solid slury of 60Si2Mn springthe slurry roling experiments were conducted on our designedsteel are discharged every time.equipment. The mill machine is irreversible single groove millFrom the above experiments， the primary austeniticmachine with two hollow rollers and the rolling groove be-grains of 60Si2Mn spring steel will become spherical or nearlytween the rollers was 5 mm wide. The rollers were 240 mm in spherical on the electromagnetic stirring condition, which maydiameter, made of cast iron and cooled by inner water. The be related with the following reasons. First, the temperaturesurface temperature of the roller was about 50°C and theirand solute field of the melt stirred by electromnagnetic field arelinear rolling speed was 1825 mm per minute.made more homogeneoush, the whole melt is almost cooledThe microstructures and the mechanical properties of theto the same nucleation temperature meanwhile and the wholesemi-solid rolled plate of 60Si2Mn spring steel in ambientareaotthemelt almost nucleaeverywhere so that laarge。se1iLnrperature were determined. The tensile samples were cut fromaustenitic dendrites are prevented and fine rose-like primarythe steady lengthwise part of the rolled plate and their size isaustenitic grains separate from the melt. Second, these fineshown in Fig.2.rose-like primary austenitic grains with the liquid come to the)中国煤化工400ufHCNMH(Fig.3 Microstructures of semi-solid 60Si2Mn spring steel strred by electromagnetic feld for 60s (a), 90 s (b),120s (c) and 120s (d)J. Mater. Sci. Technol, Vol.19 No.6, 2003615Table 1 Properties comparison between the semi- solid rolled plate and lradi-tional hot-rolled plate of 60Si2Mn spring steelRolling processSample marked Maximumn puling Rupture Sample ruptured Elongationlength Lo/mmforce /kNstrength/MPalength L/mm/9Traditional repeated hot19.7912.20.994.422.413.20rollingOne single- time semni-solid19.838.38680.321.809.56_rolling(a20C.m20CuenFig.4 Microstructures of semi-solid rlldl plate of 60Si2Mn spring steel in the center of the plate (2) and nearthe edgc of the plate (b)warmer central area one moment and go to the cooler pe-ripheral area of the stirring chamber the next, so the vi-olent temperature fuctuation of the fne ros-like primaryaustenitic grains is induced by this flow motion and acceler-ates the remelt of a large amount of the second arms on theirroot. Third, the secondary arms separated from the rose-likeprimary austenite collide and rub each other or collide andSample positou| Rllingdrotionrub against the liquid. The primary austenitic grains becomespberical in the end. .Fig.5 Position of the draw samples cut from the rolled plate3.2 Roll of semi-solid slurryThe semi-solid slurry with different solid fraction ofgroove, the slurry is obliged to flow along the shaft direc-60Si2Mn spring steel is brought to the rolling groove betweention of the rollers and is formed to the plate. Upon rolling,the two rollers of the mill machine and is rolled into a platethe flowing resistance of the spherical solid austenitic grainsthat is 120 mm wide and 5 mm thick. The surface quality ofand the liquid phase are diferent. The viscosity of the liq-the rolled plate is good and the outward appearance is neatneat uid phase is low and the flowing resistance is small, so thegenerally, however, there are some small cracks on the plateliquid in the inner area of the shurry would easily flow outsurface sometimes. If the slurry is exported too fast from thethrough the lttle gap among the solid grains. The sphericalstirring chamber, the slurry will pile up on the top of thesolid austenitic grains would collide each other on fowing androllers and can flow away from the rectangular groove so thatthe fAowing resistance is high, so that the flowing speed of thethe burrs in the direction of width often appear. The mi-spherical solid auustenitic grains is slower than the liquid, andcrostructures of the plate cross section are detected, as shownthe phenomenon of solid and liquid separation emerges afterin Fig.4. It can be seen from Fig.4 that the original sphericalrolling. The lower the linear speed of the rollers, the lonsolid grains and liquid phase in the plate are separated andthe flowing time of the liquid and the more the segregation,accumulated after rolled, that is the spherical solid grains areso the semi-solid slurry should be rolled in proper speed anddistributed in the central area and the liquid phase is aroundsolid fraction.the periphery of the plate.e microstructureregaticin the plate always exists no matter how much or lttle thesolid fraction of the slurry is. The less the solid fraction, the3.3 Mechanical properties of the rolled platemore the original liquid segregation in the plate. The mi-The tensile samples of 60Si2Mn spring steel are cut fromcrostructure segregation in the plate is decreased along with the steady lengthwise part of the rolled plate, as shown inthe solid fraction increase.. When the golid fraction of the Fig.5. The tension experiments are completed at room tem-60Si2Mn spring steel slurry is low, the shape of the spherical perature and the results are shown in Table 1. Compared withsolid austenitic grains is not changed upon rolling, as shown the properties of the traditionally repeated hot rolled 60Si2Mnin Fig.4(a). However, when the solid fraction of the 60Si2Mnspring steel, the properties of the 60Si2Mn spring steel platespring steel slurry is high, the shape of the spherical solidrolled only once in semi-solid state are decreased. The reasonsaustenitic grains is changed upon rolling.may be that. the microstrncturrs in the rolled plate are not ho-Before the semi- solid slurty of 60Si2Mn spring steel is moger中国煤化工osmall. Ifthe plateisrolled, it is transported from the stirring chamber and the rolled>le to increase the me-diameter of the slurry is about 16 mm, but the rolling grooveYHC N M H Gled plate. Moreover, ifwidth is only 5 mm, so that the slurry is compressed and flls some process measures are Taken to reduce the segregation ofin the rectangular groove. Because the rolled plate blocks the spberical primary solid austenite in the rolled plate, it isup the groove from the bottom and the slurry is on the top also probable to promote the homogeneity of the mechanical616J. Mater. Sci. Technol, Vol.19 No.6, 2003properties.REFERENCES4. Conclusions[1 I D.B.Spencer, R.Mehrabian and M.C.Flemings: Metal Trans,1972, 3A, 1925.(1) When the stirring chamber is preheated to 1350°C,[2] M.C.Flemings: Metall. Trans. A, 1991, 22A(5), 957.and the tapping temperature of the liquid 60Si2Mn spring(3] M.Garat and S.Blais: In the 5th Int. Conf. on Semi-Solid Pro-cessing of Alloys and Composites, eds. A.K. Bhasin, J.J.Moore,steel is 1600°C, and the stirring time is 2 min on the givenK.P. Young and S.Midson, Golden, Colorado, USA, June 23-25,power condition, the proper semi -solid slury with 50~60 vol.pct primary solid austenitic grains can be prepared, the size4] P.Eisen and K.P.Young: In the 6th Int. Conf. on Semi-Solidof the grains is about 100~300 pum, so it is easily dischargedProcessing of Alloys and Composites, eds. G.L.Chiarmetta andfrom the chamber.M.Rosso, Politecnico Di Torino, Turin, Italy, Sept 27-29, 2000.(2) The semi-solid slurry of 60Si2Mn spring steel is suc-[5] J.M.Oblak and W.H.Rand: Metall. Trans. B, 1976,7B(12),cessfully rolled to plate, but the original spherical primaryaustenitic grains separated with the original liquid after[6] J.M.Oblak and W.HRand: Metall. Trans. B, 1976, 7B(12),rolled, and the spherical solid grains are distributed in the[7] D.A.V.Cleave: lron Age, 1977, 220(8), 34.central area and the liquid phase is around the periphery ofthe plate. Moreover, the less the solid fraction, the more se-[8 ] K,P.Young, R.G.Riek and M.C.Flemings: Metals Technology,1979, 6(4), 130.rious the original liquid segregation in the plate(3)Comparedwith the properties of the traditionally re-9] P.Kapranos, D.H.Kirkwood and C.M.Sellars: In the 4th Int.Conf. on Semi-Solid Processing of Alloys and Compositions,peated hot rolled 60Si2Mn spring steel, the mechanical prop-eds. D.H.Kirkwood and P.Kapranos, the Universiy of Shefield,erties of the 60Si2Mn spring steel plate rolled only once inEngland, June 19-21, 1996.UK: the Department of Engineeringsemi-solid state are decreased.Materials, University of Sheffield, 1996, 306.[10] K.E.Blazek, JE.Kelly and N.S.Pottore: ISIJ International,1995, 35(6), 813.AcknowledgementThe authors would like to thank the National Natural Sci-1] Weimin MAO, Aimin ZhAO, Chengin CUI, Feng SUN, Chenglience Foundation of China for financial support under the grantZHANG and Xueyou ZHONG: Acta Metall. Sin, 2000, 36(5),No.9995440.539. (in Chinese)Effect of Carbon on DA718 Alloy with P AdditionNa Lrt, Shouren GUO, Dezhong LU, Wenru SUN, Yan XU and Zhuangqi HUInstitute of Metal Research, Chinese Acadermy of Sciences, Shenyang 110016, China[ Manuscript received October 17, 2002, in revised form December 16, 2002]Lower content of carbon can further improve the stress rupture life of p modified DA 718 aloy up to more than 270%.Meanwhile, the ductility of the allay decreased a ittle. More boron atoms dissociate due to decreasing carbon content andinteract with phosphorus which brings the longer stress rupture life of the alloy. Less carbon may induce more phosphorussegregating in the grain boundary and result in brittleness.KEY WORDS: DA 718 alloy, Carbon, Phosphorus1. Introductionfurnace and remelted in 25 kg vacuum induction furnace toget three 10 kg ingots with diferent P and C contents. TheIt was demonstrated to be true by many researchersl1~6lchemical composition of the Inconel 718 master alloy was: Nithrough their experiments that the addition of proper amount53.10; Cr 18.92; Nb 5.35; Ti 1.00; Al 0.56; Mo 3.30; Si 0.07;of phosphorus into some wrought Ni-base superalloys such asMn 0.02; S 0.002; B 0.005; Fe bal. in mass fraction. TheIN718 lloy could improve its stress-rupture and creep prop-phosphorus and carbon contents of the test alloys were an-erties, and nearly have lttle efect on their tensile strength,alyzed and listed in Table 1. After being homogenized bythe treatment of 1160°Cx8 h+1190°Cx8 h, the ingots weretoughness and other propertiesBut for IN718 alloy, boththe composition and the processing techniques are quite com-forged into 40 mm square bilets at 1120*C, and then rolledplex, many factors can affect P behaviors in the alloy-into 18 mm diameter bars at 1090°C. The rolled bars wereCarbon is a kind of beneficial minor elements in IN718then subjected to the following DA heat treatment: aged atalloy. Proper amount of carbon can strengthen the alloy by720°C for 8 h, furnace cooled at 50°C/h to 620°C, and atinterstitial solid solution, and forming fine carbides. The re-620°C for 8 h, followed by air cooling. The test alloys weresearch showed that, decreasing the amount of carbon and in-machined into stress-rupture specimens with the gage sectioncreasing P and B contents were beneficial to prolong the stressof 5 mm in diameter and 25 mm in length.rupture life of the IN718 alyls. But too low the amount ofcarbon may induce insuficiency to deoxidize and to desulfu-Table 1 Contents of phosphorus and carbonin test alloys (mass fraction)rize during casting, and these elernents may segregate to thegrain boundary to increase the brittleness of these alloys.Alloy No.experiments were done to study the effect1I1 paper,ex0.0050.03of carbon on P-modified DA (direct aging) IN718 alloy.0.0222. Experimental Procedures中国煤化工-0.01T'b IHC N M H Gby using optical mi-The master alloy was melted in 200 kg vacuum inductioncroscopy, scannng electron mcroscopy (SEM) and transmis-↑Ph.D. Candidate, to whom crrespondence should be adressed, sion electron microscopy (TEM). The fracture surfaces wereE- mail: nli@imr.ac.cn.