| Prehardened mold steel for plastic is the primary for manufacturing high quality plastic mold. With the rapid development of mold industry in our country, this kind of steel exhibits extensive application prospect. However, because of property deficiencies of domestic produced steel, import steels hold major part of market share. In order to change the situation, using "method of accelerated maturation of materials" in this article, the composition and technology of prehardened mold steel for plastic are optimized and designed by Thermo-Calc & DICTRA thermodynamic software package. On the strong background of industrial application, the property of domestic produced steel is effectively improved at the premise of no incremental cost, which reaches to the performance level of the same kind of high quality steel in the world, e.g. ASSAB718 steel. In addition, on the basis of research results mentioned above, the new kind of free machinability steel with high performance and better application is developed.With the compared and studied in this article, it is shown that the mechanical property at room temperature between import prehardened mold steel for plastic with large section and domestic produced steel is similar. Combined with microscope and SEM&EDX analyse, it is shown that the content of oxide inclusion in domestic produced steel is higher and mainly composed of aluminum oxide, but the non-metallic inclusion in ASSAB718 steel is mainly composed of soft phase sulphide inclusion in the form of manganese sulphide with aluminum oxide at the core. The flank wear amount, cutting force and the surface roughness of machining samples are comprehensively tested by turning test at low cutting speed and polishing test, it is shown that the main performance difference between domestic produced steel and ASSAB718 steel is machinability. Because of the key to affect machinability of steel is non-metallic inclusion, machinability of steel can be improved by the deformation and decreasing abrasion effect of sulphide during machining. The machinability of domestic produced steel is markedly decreased by severe abrasion effect of aluminum oxide with highhardness and high melting point on the carbide tool. So it is the main means to improve steel's performance that remove or modify the oxide inclusion in steel.SLAG database based on Gaye model and thermodynamic computation system based on CALPHAD technique in Thermo-Calc & DICTRA software package are powerful tools for computation equilibrium relation of multicomponent and multiphase system during steelmaking process. According to computational results, the precipitation process and its influencing factors of non-metallic inclusion are predicted. It is shown that the irrational composition ratio is the main factor to result in higher content of aluminum oxide inclusion in domestic produced steel by the estimation of inclusion type precipitated in steel. Then the composition of the steel is optimized, it is shown that the weight percent of alloy elements carbon, manganese, silicon, chromium, nickel and molybdenum are at the range of 0.36~0.42wt%, 1.35~1.45wt%. 0.2~0.3wt%, 1.8~2.0wt%, 0.8-1.0wt% and 0.2~0.4wt% respectively. The experimental results show that the content of aluminum oxide inclusion is markedly reduced and mainly substituted by MnS with minuscule aluminum oxide particle at the core, the machinability of domestic produced steel after composition regulation is markedly improved.Due to the close relationship between phase transformation and heat treatment technology establishment, the main type of carbide in prehardened mold steel for plastic is M7C3 type by TEM analyse and computed results. Then the effect of temperature and alloy composition on the dissolution of M7C3 type carbide in steel is simulated by"Cell" model in DICTRA program. It is shown that generally the dissolution contents of M7C3 type carbide are increased with enhanced heating temperature and prolonged keeping temperature time, the relationship between dissolution content of M7C3 type carbide and time at the temperature of 1183K(910°C) is close to that at the temperature of 1203K(930°C). Alloy elements Cr> Mn> Mo affect the dissolution of M7C3 type carbide. According to simulated results, the segregation of elements in large module is basically eliminated by high temperature homogenizing treatment before forging, that is done by keeping 10 hours at the temperature of 1200+10 °C, thus surface roughness ofmachining steel sample is improved. In addition, the cracking of module with small thickness results from quenching is avoided substituted by 910°C normalizing, the microstructure and property of the steel are not basically affected.The performance of domestic produced steel with large section after composition regulation and homogenization treatment at high temperature before forging, including machinability, polishing properties and anti-corrosion performance, reach to the level of ASSAB718 steel and it can be produced in mass.It is helpful to improve the machinability of steel with addition of free cutting elements such as sulphur, calcium, etc. But precipitation of harmful non-metallic inclusion, such as calcium aluminates and calcium sulphide, can be avoided only by regulation the content. The effect of elements S, Ca with different contents on the precipitation of non-metallic inclusion is computed by Thermo-Calc software package, it is shown that perfect free cutting phase can form in steel at the proper composition range, such as glassy oxide inclusion 2CaO·Al2O3·SiO2 (that is C2AS) . Free machinability prehardened mold steel for plastic with modified composition is tentatively produced. Combined with SEM&EDX analyse, it is shown that free cutting phase C2AS can be obtained by addition 20~40ppm calcium and 0.02wt% sulphur, but C2AS is not easy to be obtained if the content of sulphur is increased to 0.04wt%, that is agree with the prediction.Compared with cutting at low speed, it is shown that at high cutting speed, the damage of tool is heavy and tool life is short, but the cutting force becomes small and the surface roughness of machining surface is also improved. The existence of soft phase sulphide results in better properties of steel at low cutting speed condition, but at high cutting speed condition, only the glassy oxide C2AS is testified to form protective layer at the surface of tool, then the wear of tool is effectively inhibited and the machinability of steel is improved. In addition, the free machinability effect of sulphide phase can be improved by addition of calcium and bismuth because of changing sulphide's shape and increasing the stick strength. The ductility and isotropy of steel are decreased because of the deformation of soft phase sulphide, but the modified effect of...
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