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AbstractPlastic mould steels are the most versatile alloy steels whose consumption amount is thelargest among mould and die steels. Pre-hardened plastic mould blocks are mainly used formanufacturing large moulds of automobile interior trimming products and shells of large-sizehousehold appliance, etc. The metallurgical processes for large-size pre-hardened mouldblocks of steel 718 was developed by Central Iron & Steel Research Institute (CISRI) andDongbei Special Steel Group Co., Ltd. (DSSC) together and the effects of relative processes,such as LF ? VD melting process, ingot mold design, 27789
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argon protective ingot casting process,high temperature homogenizing process and forging process for heavy ingots, quenchingprocess and high temperature tempering process for mould blocks on material purity andhardness uniformity were systematically tested. Results show that sulfur content can becontrolled as low as 0.005%, and total oxygen content as low as 12 ppm if slag basicity of LFand VD were kept within 3.5–4.0 and 3.0–3.5 respectively. Rejection rate caused byinclusions at ingot bottom was reduced from 6.81 to 1.55% by optimizing the design of the28 t ingot mold bottom taper shape and ingate chamfering. It was confirmed by argonprotective casting test that argon flow rate should be controlled at 4–8 m3/h. The macro-segregation and hardness deviation were improved from Bgrade 3.0 and B5.0 HRC toBgrade 2.0 and B3.5 HRC respectively by adopting high temperature homogenizing process.The macro-porosity and the ultrasonic testing result was improved from Bgrade 3.0 and C/dto Bgrade 2.0 and D/d respectively by applying FM forging method instead of drawing by flatanvil. The hardness deviation on the cross section of 650 mm 9 1,080 mm mould blockswas B3.5 HRC with pre-hardening treatment i.e. water–air alternatively timed quench-ing ? high temperature tempering by using electrical heating furnace.KeywordsPlastic mould steels Pre-hardened mould block Hardness Microstructure Mould Industry Association, total consumption amount ofmould steels in 2007 is 700,000 t in China, and this figure isincreasing at a rate higher than 12% per year. It was pre-dicted that mould steel output in 2010 is up to 1,000,000 t,and plastic mould steel, cold-working mould steel, hot-working mould steel, and special property mould steel are inthe proportion of about 5:2.8:2:0.2 [1–3].USA established P-series of plastic mould steels in 1960sand P20 was a representative steel grade of the series. LaterSweden developed 718 (P20 ? Ni), whose hardenabilitywas improved by adding Ni to P20 steel and it was desig-nated for making large-size moulds. The standardizationand serialization of mould steels promote the developmentof relative technologies and equipments. With the devel-opment of new type plastics and the extension of theirapplication field, the research of special-purpose plasticmould steels was intensified. Different kinds of plasticproducts require different properties of mould steels. As aresult, many series of special plastic mould steels weredeveloped, including carbon structural steels, carburizingplastic mould steels, pre-hardened plastic mould steels, age-hardening steels, and corrosion resistant steels [4, 5].In early 1980s, when Chinese mould and die toolindustry still lagged behind and almost all plastic mouldsare made of carbon structural steel. Many problems wereencountered, such as short service life of mould, poor sur-face condition of machined mould cavity, and low qualityof pressed plastics. In order to improve domestic materialquality, plastic mould steel P20 (3Cr2Mo) in Americanstandard was introduced by Chinese enterprises, and laterincluded into Chinese national standard (GB1299-85).Afterwards, the Sweden 718 steel was included in GB/T1299-2000 [6, 7]. According to YB/T1299-1997, steelgrades and their chemical compositions of plastic mouldsteels are shown in Table 1.In order to meet the demands of domestic markets onhigh-quality mould steels, Chinese special steel enterprisesstarted to introduce advanced metallurgical equipments andrelative technologies in early 1980s. Through implementa-tion of state-level key science technology projects fordevelopment of mould steels through ‘‘the sixth five-year’’plan period (1981–1985), ‘‘the seventh five-year’’ planperiod (1986–1990), and ‘‘the eighth five-year’’ plan period(1991–1995), Fushun Steel Company (current FushunSpecial Steel Co., Ltd. of DSSC) and Shanghai Fifth SteelCompany (current special steel pision of Baosteel) finallybecome representatives of mould steel production base inChina. Both special steel works possess advanced equip-ments such as UHP EAF, large ESR furnace, fast forgingpress, rotary forging machine, high-precision flat rollingmill, continuous rolling lines, and special heat treatmentlines and are capable of manufacturing mould steels as perany advanced standard. But the overall quality level ofChina made mould steels at that time still lagged behindinternational advanced level. There were problems whichneed to solve with domestic mould steels could be describedgenerally as follows [8]: (1) The variety of domestic mouldsteels and size of mould steel products were incomplete; (2)There lacked mould steels of high quality, high stability,and high level properties; (3) The proportion of mouldblocks and near-net-shape products among mould steelproducts is relatively small; (4) The appearance of steelproducts was not up to the standard.According to customs statistics at that time, China nee-ded to import lots of moulds every year, especially somelarge-size, complex, precise moulds with long service life.The variety, size, and quality stability of domestic mouldsteels could not meet the requirements for manufacturingtop-grade moulds. P20 and 718 pre-hardened plastic mouldblocks used for manufacturing moulds for large householdappliances could not be produced at that time. For example,plastic mould steels used for manufacturing moulds of largecolour TV cabinet were only imported. According toincomplete statistics, only plastic mould enterprises in thePearl River Delta Region (about 100 enterprises, includingSino-foreign contractual joint ventures) need to import3,000–5,000 t top-grade plastic mould steels, whose price isas high as 55,000–60,000 RMB/t. There was a commoneagerness of Chinese mould industry and special steelenterprises that the quality level of mould steels needs toimprove, and high-quality plastic mould steels need todevelop.In 1980s and early 1990s, relative research institutes,special steel companies, and some mould enterprises cametogether starting a project named ‘‘technical development ofplastic mould steel blocks and bars’’. During this period, theeffects of chemical composition and metallurgical processon the microstructure and mechanical properties of P20 and 718 were systematically studied. As a result, the specifica-tions and technical procedures of these steels were estab-lished. In the meantime, some special plastic mould steelgrades such as P20B, P20BSCa, 8Cr2MnWMoVS, and5NiSCa [9] were developed by some Chinese universitiesand research institutes independently. However, the largescale industrialization of these steels was not realizedalthough these steels show good and unique properties.In mid and late 1990s, pre-hardened plastic mould steelswere anticipated by mould manufacturers, because for high-precision plastic moulds with complex cavities, heat treat-ment after processing leads to defects of complex cavities,such as quenching deformation, cracking and decarburiza-tion. Pre-hardened plastic mould steels are mould steelproducts or mould blocks quenched and tempered by steelmaking companies, and this kind of material can bemachined into moulds directly by customers without furtherquenching and tempering treatment. Pre-hardened mouldsteels are suitable for manufacturing large-size and middle-size precise moulds with complex shapes, or for massproduction [10]. The carbon content of pre-hardened plasticmould steels is generally controlled within 0.3–0.5 mass%,and some alloy elements such as Cr, Mo, Ni and V areadded. Pre-hardened plastic mould steels mainly includespecial developed steels such as P20 (3Cr2Mo), 718(3Cr2NiMnMo) as per the requirements of plastic moulds,and some general alloy structural steels and hot-workingdie steels such as 40Cr, 42CrMo, 5CrNiMo, 5CrMnMo,4Cr5MoSiV, etc. P20 and 718 are universally used for pre-hardened plastic mould steels, which are mainly used forinjection moulding of all kinds of thermoplastics such aspolyformaldehyde, nylon, polyethylene, polypropylene, andpolyvinyl chloride.Good grinding, polishing, surface finish, and weldabilityare key properties for pre-hardened plastic mould steelsbesides the strength and toughness requirement. High pur-ity, structural uniformity as well as the well-controlledchemical composition are the main factors in guaranteeingthe above-mentioned properties. At present, melting pro-cesses used for plastic die steels in China are double meltingprocesses, i.e. EAF/convertor/induction furnace/vacuumfurnace ? refining melting (vacuum refining melting orspray forming) or secondary remelting (ESR or VAR).Ingot casting is still used for these mould steels. The ingotsare forged or rolled at certain deformation ratio and thenheat-treated as per required delivery conditions. It is sig-nificant to systematically study the processing factors thatcan affect the metallurgical qualities of large-size pre-hardened plastic mould blocks.Literatures [11–13] studied the effect of temperedmicrostructure, non-metallic inclusions, and [S] on materialpolishing property. Their results showed that the micro-structure should be controlled as tempered martensite andtempered bainite, non-metallic inclusions should be Bclass1.0, and [S] should be controlled20.005%.Literature [14] studied the band microstructure due tochemical composition segregation, which can cause hard-ness inhomogeneity, and affect machined surface quality.The effect of high temperature homogenizing treatment oncompositional segregation was also discussed.Literatures [15, 16] studied the effect of heat treatmenton the tempered microstructure of mould blocks, as well asthe correlation between microstructure and hardness.Quenching ? high temperature tempering is a commonpre-hardening process for large-size steel 718 mould blocks.For mould blocks with simple configuration, both water–oildouble quenching and water quenching can be used. But forlarge-size mould blocks, oil cooling cannot guarantee thehardness uniformity on cross sections due to its limitedcooling capacity, while water cooling is inclined to causequenching cracks. Water–air alternatively timed quenchingsoftware was designed by Shanghai Jiaotong Universitythrough numerical simulation of the time related changes oftemperature field, microstructure field, stress/strain field andproduct properties. With the help of this software, themould blocks can be quenched in accordance with the pre-set program [17].2 Research on the MetallurgicalTechnology of Plastic Mould SteelsIn recent 10 years, the metallurgical technology relating tolarge-size mould blocks such as LF ? VD melting process,ingot mold design, argon protective casting, homogenizingannealing and forging process for heavy ingots, quenchingprocess and high temperature tempering process were sys-tematically studied by many research groups from CISRI,DSSC and some colleges and universities. A lot of pro-cessing parameters and related technical patents haveacquired and applied. This paper mainly introduced theresearch achievements on large-size pre-hardened mouldblocks of steel 718 done by DSSC and CISRI.2.1 Technology for Controlling MaterialPurityStudies show that globular brittle inclusions and strip-likesulfide in materials are inclined to flake off during polishing,resulting in defects such as ‘‘pitting corrosion’’ and ‘‘orangeskin’’. Therefore, the improvement of material purity,reduction of non-metallic inclusions, as well as the reduc-tion of T [O] and [S] content in steel are the key factors indeveloping high quality plastic mould steels. UHP ? LF ? VD ? ingot casting is the common met-allurgical processing route used in domestic special steelindustry.2.1.1 Effect of LF 1 VD Process Parameterson Material PurityLF ? VD refining melting is the key process for desul-phurizing and deoxidizing. During LF refining, the whiteslag inside LF is under low oxygen atmosphere, and argonstirring speeds up the reactions at slag–liquid steel interface.At the same time, the temperature of the molten steel wascompensated by arc heating, which can guarantee enoughrefining melting time, and lower oxygen and sulphur con-tents. Tables 2 and 3 show the effects of slag basicity ondesulphurizing and on deoxidizing in LF and VD furnacerespectively.The data in Tables 2 and 3 show that T[O] and [S] con-tents can be effectively controlled through controlling whiteslag basicity. When the white slag basicity in LF wasincreased from 2.0–2.5 to 3.5–4.0, desulphurizing rate wasincreased from 47.37 to 72.22%, and deoxidizing ratewas increased from 47.37 to 72.22%. When the same heatwas melted in VD furnace and vacuum degree is 67 Pa, whenslag basicity increased from 2.0–2.5 to 3.0–3.5, the desul-phurizing rate increased from 45 to 70%and deoxidizing ratefrom14.29 to 20%respectively. Trial results show that sulfurcontent is 0.003%, and total oxygen content is 12 ppm afterbreaking the vacuum of VD furnace if slag of high basicity(3.0–3.5) is used. It means that the steel purity conforms tospecified requirements.2.1.2 Effect of Ingot Mold Design Parameterson Steel PurityBottom casting is usually used for high quality mould alloysteels. At present, the height of the trumpet used for large-size ingot casting system is generally within 3–3.5 m.During casting, vortex is easy to form at the mold bottomdue to great hydrostatic pressure from the liquid in trumpetand the effect of the mold ingate shape, which not only cancause slag entrapment, but also can affect material purityand yield in some serious condition. In order to avoid slaginclusions, a reasonable mold design parameter was fixedthrough simulation research of the shape of mold ingate.Figure 1 is the scheme of fluid flow pattern into the ingotmold bottom both before and after optimizing the molddesign, and data in Table 4 are the ultrasonic testing resultof the die blocks corresponding to the mold design. Inclu-sion rejection rate of 28 t ingots was reduced from 6.81 to1.55% by adopting new type ingot mold, and test resultsshow that the design of the new taper and the ingatechamfer was reasonable, by which the vortex at the bottomwas limited effectively.Table 2 Effect of white slag basicity on T[O] and [S] content in LFWhiteslagbasicity[S] at thebeginning(%)[S] atthe end(%)T [O] at thebeginning(ppm)T [O] atthe end(ppm)2.0–2.5 0.038 0.020 50 213.5–4.0 0.036 0.010 55 15Table 3 Effect of slag basicity on T[O] and [S] content in VD furnaceSlagbasicityVacuumdegree(Pa)[S] at thebeginning(%)[S] attheend(%)T [O] atthebeginning(ppm)T [O]at theend(ppm)2.0–2.5 67 0.020 0.011 21 183.0–3.5 67 0.010 0.003 15 12 2.2 Effect of Argon Protection on MaterialPurityExogenous inclusions come mainly from reoxidation ofmolten steel during casting. They generally exert deleteri-ous effect on mechanical properties and the life of steelmoulds. It is known that protective measures during castingcan improve material purity effectively. This work studiedthe effect of argon protection by measuring nitrogen contentin steel before and after casting.Figure 2 shows the increased nitrogen amount versusdifferent argon flow rate from 2 to 10 m3/h during casting of192 heats. The secondary data fitting curve and expression(1) was established on the basis of the testing data byapplying data regression method in Minitab statistic tool.Expression of data fitting curve is as follows:DN ¼ 1:46Q2Ar 18:85QAr þ 69:03 ð1Þwhere DN, Nitrogen mass fraction change before and aftercasting (910-6); QAr, Argon flow rate when argon protec-tion equipment is at work (m3/h).According to expression (1), the smallest increasednitrogen amount is (8.19 9 10-6) when argon flow rate iscontrolled at 6.45 m3/h that is within the capability of ourcurrent argon protection equipment. So it was finally con-firmed that the argon flow rate in the range of 4–8 m3/h isthe optimal value in accordance with our current processcondition.Table 4 Effect of mold bottom shape on ultrasonic testing resultMold Weight (t) Inclusion reject rate (%)Original mold 289 6.81New mold 320.15 1.55 2.3 Controlling Techniqueof the Microstructure Uniformityof the Mould BlocksFor plastic moulds whose working conditions are rathersevere, the uniform microstructure is beneficial to the ser-vice stability of the moulds.2.3.1 Effect of High TemperatureHomogenizing Process on MicrostructureHomogeneityGenerally, chemical segregation is unavoidable for large-size ingots during solidification. The segregation leads tothe banding microstructure and exerts deleterious effect onmaterial transverse properties. In order to analyze the effectof chemical segregation on the microstructure of pre-hard-ened mould blocks, a mould block whose thickness is810 mm was cut into several pieces, and samples weretaken to represent the edge, 1/8, 1/4, 3/8 thickness positions,and the center of the mould block. Figure 3 shows thelongitudinal macrostructures of the samples after etched by4% Nital for 10 s. It is seen from Fig. 3 that the macro-structures of the samples taken from edge and 1/8 of themould block (1 and 2 in Fig. 3) are fine and uniform, whilethose of the samples of 1/4, 3/8 and the center (3, 4 and 5 inFig. 3) are not uniform. The specimens were further ana-lyzed by microscopic inspection as shown in Figs. 4 and 5.In Fig. 4, the dark area was inspected with high magnifi-cation further and the microstructure resembles martensiteand/or low bainite at 1/4 thickness position. In Fig. 5, moreprominent martensite is shown at the center of thickness.The microstructural inhomogeneity as shown in Figs. 4and 5 is relating to the element microsegregation. The localalloy element concentration was analyzed by using electronmicroprobe. Figure 6 shows the abnormal microstructureregion A and B, and also the normal microstructure regionC and D. Microprobe results in Table 5 show that Mn and Ni segregation is rather small, while Cr and Mo segregationis relatively serious. The maximum content of Mo is eventhree times than the minimum content.The as-quenched hardness uniformity and microstruc-tural homogeneity can be improved by improving the dis-tribution concentration of alloy elements. High temperaturehomogenizing treatment is an effective way to reduce theelements segregation. Table 6 shows the macro-segregationgrade (according to ASTM A 561) and the hardness devia-tion of mould blocks forged from ingots undergone normalheat treatment and high temperature homogenizing treat-ment respectively. It can be concluded that ingot segregationand hardness uniformity can be improved remarkably byusing high temperature homogenizing treatment.Figure 7 shows the micrograph of large-size mouldblock. At the block edge there is tempered martensite, at 1/4thickness and thickness center there is mainly temperedbainite. No granular bainite microstructure was found thatproves the high temperature homogenizing treatment ofheavy ingot is effective.