Semisolid Slurry Preparation of Die Steel with High Chromium Content

Vol.11 No. 5J. Iron &. Steel Res.，Int,Sep. 2004 :Semisolid Slurry Preparation of Die Steel with High Chromium ContentMAO Wei-min,ZHAO Ai-min,ZHANG Li-juan,ZHONG Xue-you(University of Science and Technology Beiing，Beijing 100083, China)Abstract: The semisolid slurry preparation of die steels Cr12 and Cr12MoV with high chromi-um content was studied. The results show that the semisolid slurry of both steels with solid of40 %-60 % can be made by electromagnetic stirring method and is easy to be discharged fromthe bottom little hole of the stirring chamber. The sizes of the spherical primary austenite in theslurry of die steels Cr12 and Cr12MoV are 50一100 μm and 80一150 μm, respectively. Thehomogeneous temperature field and solute field for both steel melts are obtained. The strongtemperature fluctuation in the melt with many fine primary austenite grains occurs and theremelting of the secondary arm roots at the same time is accelerated because of the electromag-netic stirring. These are the most important reasons for deposition of spherical primary austen-ite grains.Key words: high chromium die steel; semisolid; electromagnetic stirringThe semisolid forming technology of metals or in continuous semisolid cast billet of alloys with hightheir alloys has been studied deeply in the earlymelting point by electromagnetic stirring was a1970s. The experiments show that the new technol-chieved by K E Blazek et al[6]. But, the main tend-ogy has many advantages, such as reducing porosityency of the semisolid metal forming is to developand solidification shrinkage in the formed castings,proper rheocasting technology and decrease the pro-and the compactness and strength of metal or alloyduction cost， because the semisolid thixocastingare increased. Because of the reduction of composi-process is longer and the casting production cost istion segregation and deformation resistance and thehigher. For semisolid iron or steel rheocasting， theimprovement of the property uniformity, the sophis-mechanical stirring is not proper for preparation of i-ticated castings can be formed. Owing to the factron or steel slurry for short mechanical stirringthat the yield is increased, the rejection rate and thechamber and stirrer life and easily polluting the slur-forming temperature are reduced. Meanwhile, thery. The SIMA and Ospray methods are only usedlife of die is prolonged and the production is prone tofor preparation of semisolid billets. Therefore， therealize automatization. The semisolid forming tech-electromagnetic stirring may be the important meth-nology of aluminum alloys and magnesium alloys hasod for continuously producing the steel slurry. Upbeen successfully used to manufacture many parts ofto now，however, the research on the production ofcars and other machine parts[1-3] .steel slurry using electromagnetic stirring method isWhile the semisolid forming of alloys with lowscarce because the electromagnetic stirring methodsmelting point was studied, the semisolid forming olare mainly used to produce semisolid billets of alu-ferrous alloys has also been researched on a certainminum alloys. The high chromium die steels Cr12scale. M C Flemings and his fellows of MIT pre-and Cr12MoV are two kinds of excellent die materialpared the semisolid slurries or billets of AISI 440C,for higher carbon content and wider solidification304，4340 steels with mechanical stirring equip-temperature range， so they are easier to preparements, did the semisolid die casting experiments andsemisolid slurry and improve inner quality with refi-obtained compact castings properly-1.4]. P Kapranosning solidified crvstals In this paper，the micro-et al，also produced the semisolid billet of highstruct中国煤化Iisolid slurries of highspeed steel M2 by Ospray or SIMA methods and car-chromiY片C N M H GCr12MoV were stud-ried out the forming experimentsb5l. Some progressied by electromagnetic stirring， which can provideFoundation item: Item Sponsored by National Natural Science Foundation of China (59995440)Biography: MAO Weir min (1958-)， Male, Doctor, Professor; Received date: December 24, 2002No.5Semisolid Slurry Preparation of Die Steel with High Chromium ContentSep. .experience and experimental basis for rheocastingFig.1. As the superheated liquid high chromium dieprocess of steel slurry with high melting point.steel Cr12 and Crl2MoV are poured into the stirringchamber, it is stirred right away by electromagnetic1 Experimental Methodfield and cooled continuously. When the semisolidThe raw material is high chromium die steelsmelt is stirred and the cooling rate is appropriatelyCr12 and Cr12MoV， and their compositions arecontrolled，the semisolid melt can be held at theshown in Table 1. The liquid steel Cr12 otemperature between liquidus temperature and soli-Cr12MoV of 10- 20 kg was obtained by a mediumdus temperature for enough time and the slurry offrequency coreless induction furnace. The liquidspherical or nearly spherical primary austenite grainssteels Cr12 and Cr12MoV were poured at 1 500 'Ccan be easily obtained. When the slurry reaches theand 1 600 C respectively to heat the stirring cham-given solid fraction， the center block bar of the e-ber liner before solidification. After tapping， thequipment is risen up and the semisolid slurry flowsliquid steel was poured into the stirring chamber ofout of the stirring chamber from the little bottomthe semisolid steel slurry preparation equipment andhole. In addition, the chamber and center block barstirred immediately by electromagnetic field.should be made from proper refractory which mustThe equipment for preparation of semisolidhave enough fire- resistance and thermal shock resist-steel slurry by electromagnetic stirring is shown irance. The temperatures of the chamber top and bo-Table 1 Composition of steel Cr12 and Cr12MoV%SteelSMnMoVCr122.0-2.3≤0. 40≤0.40≤0.03≤0.03 11.50-13. 00Cr12MoV 1. 45-1.70≤0. 40≤0. 40≤0.03 11.00- 12.500. 40-0.60 0. 15-0.303、2ttom are determined by Pt- Rh-Pt thermocouples andthe liquid high chromium die steel Cr12 andCr12MoV are preheated to 970 C before pouring.As the high chromium die steels are stirred for a se-4ries of given time, a small amount of semisolid steel1、slurry flows out of the chamber and is quenched inwater, and the center block bar is again lowered tothe origin place and closes the bottom hole. Metallo-graphic samples were prepared using the stirred and .quenched high chromium die steel Cr12 and。Cr12MoV. The samples were roughly and exactlypolished, etched by aqua regia, and observed withoptical microscope to analyze the shape and distribu-5tion of the spherical primary austenite grains. .The experimental preparation conditions ofsemisolid high chromium die steel Cr12 andCr12MoV by electromagnetic stirring are shown inTable 2.2 Experimental Results and Discussion中国煤化工by pouring liquid die1一Elctromagnetic stirrer; 2 一Pouring basin;3- Center block bar; 4 - Chamber;steel:YHCNMHGuldof20mmindiam-; - Quenching water pooleter. The microstructure of the sample is shown inFig. 1 Schematic of equipment preparingFig.2 (a). The primary austenite of high chromiumsemisolid steel slurrydie steel Cr12 without stirring will be dendrite crys-2004J，Iron &. Steel Res.，Int,Vol.11Table 2 Preparation parameter by electromagnetic stirring with power of 9. 78 kWSteelExperimental conditionPreheated temperature of chamber Stirring time/s170230270395450510Cr12970 CSample No. .1-11-21-31-41-51-6Preheated temperature of chamber Stirring time/s 12018023531560435Crl2MoV980 CSample No.2-2-32-42-52-6210gm! 200ym;200pnPreheated temperature- 970 C; Stirring power - 9. 78 kW(a) Os;(b) 170 s;(c) 270 8;(d) 395 8; (e) 450 s;(f) 510sFig. 2 Microstructure of semisolid Cr12 with different stirring timetal under the traditional condition， and the firstWhen the stirring time is raised further, and the sol-arms of the dendrites are very long and almost pene-id fraction is about 60 % in volume， the primarytrate the sample' s transverse section. However, theaustenite grains are very spherical, and there are al-solidification microstructure varies greatly under theso a few long primary austenite grains in the micro-condition of electromagnetic stirring and the primarystructure, as shown with arrows in Fig.2 (e) andaustenitic dendrite is changed into spherical grains,(f). Fig.2 also shows that the primary austeniteas shown in Fig.2 (b) to (f). The bigger lightgrains are refined obviously under the preparationspherical particles are the primary austenite grainscondition, the size of the primary austenite grains isbefore quenching while the smaller dark ones are Oabout 50一100 μm and the grain size is nearly notriginal liquid quenched. With electromagnetic stir-varied when the stirring time is raised because of thering for 170 s and solid fraction in the die steel Cr12constant continuous cooling rate in solidification. Itmelt of 30 % in volume，many primary austenitecan be seen from Fig. 2 that when the stirring timegrains are nearly spherical but not very round, and .lasts 5 min，the semisolid slurry of high chromiumsome of them are also fine rosette-shaped grains, asdie steel Cr12 with solid fraction of 50 % in volumeshown with arrows in Fig.2 (b). When the stirring and spherical primary austenite grains can be made,time is increased to 270 s and the solid fraction is a-and the longer stirring time， such as 7-8 min, isbout 40 % in volume, most of the primary austeniteneeded if more spherical primary austenite grains aregrains are converted to spherical grains but not verywanted, and if the cooling rate in solidification is in-round, as shown in Fig.2 (c). When the stirringcreas中国煤化Ionding stirring timetime is increased to 395 s and the solid fraction is a-may ICNMHGfthestringpowerisbout 50 % in volume, the primary austenite grainsdecreased to 4一6 kW，the spherical conversionare spherical and rounder, and there are only a smallprocess of the primary austenite grains is prolongedamount of long primary austenite grains in the miand the shape of the primary austenite grains is notcrostructure, as shown with arrows in Fig.2 (d).completely spherical. The experiments show thatNo. 5Semisolid Slurry Preparation of Die Steel with High Chromium ContentSep.when the slurry solid fractionis 40 %-60 % in vol-obviously under the preparation conditions， and theume, and the center block bar of the stirring equip-grain size of the primary austenite is 80一150 μm.ment is lifted at this time，the semisolid slurry isWhen the slurry solid fraction is 40 %- 60 %，it iseasy to flow off from the chamber bottom little hole :easy to flow off from the chamber bottom little holeunder the gravity.under the gravity. .A sample is also prepared by pouring liquid dieThe primary austenite of high chromium diesteel Cr12MoV into a metallic mould with diametersteel Cr12 and Cr12MoV will be dendrite crystalof 20 mm, and the microstructure is shown in Fig. 3without stirring and spherical or near spherical par-(a). The primary austenite of high chromium diticles with electromagnetic stirring. This phenome-steel Cr12MoV without stirring will be dendrite un- non should be related to the following real solidifica-der the traditional condition， and the first arms oftion conditions. First, the experiments showed thatthe dendrites are longer and the secondary arm spacethe temperature field and solute field of the alumi-is smaller than that of Fig.2 (a) because of fastnum silicon alloy melt stirred strongly by electro-cooling rate. Fig.3 (b) to (f) are the microstruc-magnetic field are more homogeneous7.8， so thetures with different stirring time and the same stirtemperature field and solute field of the high chromi-ring power, in which the bigger light spherical parti-um die steels Cr12 and Cr12MoV should be homoge-cles are primary austenite grains before quenchingneous too. The whole melt of high chromium diwhile the smaller dark are original liquid quenched.steels Cr12 and Cr12MoV in every test is almostIt can be seen from Fig. 3 that with the stirring timecooled to the same nucleation temperature and theincreasing，the solid fraction is increased more andnucleation happens on almost the whole area of themore and the primary austenite grains becomemelt so that the preferred growth direction of therounder and rounder. Because the pouring tempera-primary austenite dendrites is confined. The largeture is high, the chamber is heated to higher tem-primary austenite dendrites are prevented, and fineperature before solidification and the cooling rate ofprimary austenite dendrites are separated from thethe Crl2MoV steel melt in solidification is de-melt, so the basis is made for getting spherical pri-creased, the primary austenite grain size becomesnary austenite grains， as shown with arrows inlarger and more time is needed for the completeFig.2 (b) and Fig.3 (b). Second, the experimentsspheroidization of primary austenite. Even if stirringshow that the semisolid melt stirred by electromag-for 360- 435 s， there are some rosette-shaped pri-netic field is mainly turned horizontally, and mean-mary austenites in the microstructure, as shownwhile an additional flow exists， by which the meltwith arrows in Fig.3 (e) and (f). The experimentsfrequently moved up along the internal wall of theshow that the primary austenite grains are refinedchamber and then down to the center. Consequent-中国煤化工HCNMHG20uPreheated temperature- 980 C; Stirring power- 9. 78 kW(a)Os; (b) 120 s;(c) 235 s;(d) 315 s;(e) 360s; (f) 435 sFig.3 Microstructure of semisolid Cr12MoV with different stirring time-1200J，Iron &. Steel Res.，Int,Vol.11ly，these fine primary austenite dendrites with thesize of the spherical primary austenite in die steelliquid come to the warmer center area for a momentCr12 slurry is 50一100 μm and that in die steeland go to the cooler peripheral area of the stirringCr12MoV slurry is 80- 150 μm.chamber next. The violent temperature fluctuation(3) More homogeneous temperature field andis induced by this flow and accelerates the remeltingsolute field of the high chromium die steel Cr12 andof a large amount of secondary arms at their roots,Cr12MoV melt are obtained. The stronger tempera-so many spherical or near- spherical primary austen-ture fluctuation in the melt with many fine primaryite grains appear.' The necked secondary arms andaustenite grains occurs, and at the same time, ththe primary austenite from which some secondaryremelting of secondary arm roots is accelerated be-arms are remelted away are shown with arrows ircause of the electromagnetic stirring， which are theFig.2 (c) and Fig.3 (c). These necked secondarymost important reason for the deposition of sphericalarms will be continuously remelted at their roots andprimary austenite grains.separated from the primary austenite during the laterReferences :slurry stirring. Third, the stirring power is anotherimportant factor leading to the formation of the1] Flemings M C. Rheological Behaviour of Sn-15pct Pb in the .Crstallization Range [J]. Metallurgical Transactions. 1991.spherical primary austenite grains. The larger the22A(5): 957.stirring power is, the more vigorous the slurry mo-[2] Garat M, Blais s. Aluminium Semit Solid Processing: From thetion will be. 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If the stirring time is long enough,Torino, 2000. 41-46.all the secondary arms of the fine primary austenite[4] YoungK P, Rick R G, Flemings M C. Structure and Proper-will be separated finally. The microstructuresties of Thixocast Steels [J]. Metals Technology, 1979, 6(4):stirred for longer time support this viewpoint. Ir130-137.addition, the primary austenite particles will become[5] Kapranos P, Kirkwood D H, Sellars C M. Thixoforming HighPoint Alloys into Non-Metallic Dies [A]. Kirkwood D H, Ka-more spherical because the primary austenite parti-pranosP, eds. The 4th Int Conf on Semi -Solid Processing ofcles collide and rub strongly each other or with theAlloys and Compositions [C]. UK: University of Sheffield,liquid.1996. 306-311.[6] Blazek K E, Kelly J E, Pottore N S. The Development of aConclusionsContinuous Rheocaster for Ferrous and High Melting Point A-loys [J]. ISI International, 1995， 35(6): 813-818.(1) The semisolid slurry of die steels Cr12 and[7] Mao Wei-min, Zhao Ai-min, Cui Chenglin, et al. Research onCr12MoV with solid of 40 %- 60 % can be made bythe Continuous Cast Bllets of Semit Solid AISi7 Mg Alloy andTheir Microstructure Formation [J]. Acta Metallurgica Sinica,electromagnetic stirring method and discharged easi-2000，36(5): 539-544 (in Chinese).ly from the bottom little hole of the stirring cham-[8] Mao Weimin, Zhao Ai-min, Li Yamnjun, et al. The Formationber. .Mechanism of Non- Dendritic Primary ar Al Phase in Semit Solid(2) The primary austenite grains are refinedAISi7 Mg Alloy [J]. Science and Technology of Advanced Ma-obviously under the preparation conditions, so theterials, 2001, 29(1): 97-99.中国煤化工MHCNMHG.一12