| The manufacturing capacity of high-end special steel is an important indicator of a country's industrial level.In recent years,with the rapid development of China's mold industry,the demand for high-quality large cross-section plastic mold steel has surged.At present,pre-hardened plastic mold steel with a thickness exceeding 800 mm still has a large gap with the international advanced level,which needs to be solved urgently.The hardness difference of high quality large cross-section plastic mold steel should be controlled within± 1.5 HRC.Meanwhile,the mold steel have high requirement of material strength matching with toughness due to its harsh service conditions.Furthermore,no large-sized hard inclusions are allowed in substrate and the total oxygen content should be controlled below 10 ppm.Based on the above requirements and the characteristics of module size,the technical difficulties to be solved urgently are as follows.First,the relationship between the evolution of microstructure and mechanical property is deeply studied based on the composition and microsegregation characteristics of the substrate.It is necessary to develop the heat treatment method which matches the best strength and toughness and meets the requirements of the narrow hardness range.Second,the preparation technology of high purity mold steel should be developed to achieve the ultra-low oxygen control level.Focusing on the technical difficulties,this paper studies the uniformity control of microstructure during heat treatment process,microsegregation control of as-cast alloy elements,purification smelting control by Rare Earth(RE)and optimization of the alloy composition by means of experimental characterization and computer simulation.A set of common technologies for the preparation of high quality large cross-section plastic mold steel have been developed and industrialized application has been realized.The main research work and conclusions are as follows:(1)Designed and optimized the heat treatment process to achieve excellent cross-section hardness uniformity,strength and toughness based on the research of microstructure control of 718H mold steel.The results indicate that the test steel after normalizing-quenching-tempering(N-QT)process significantly improves the comprehensive properties by controlling the grain structure,dislocation density and precipitates distribution.The optimal quenching parameters of module with a thickness of 505 mm were determined by finite element simulation,i.e.,air cooling 400 s to A1 at 860?,a circulation of "water immersion 100-200 s + air cooling 100-300 s"until the final surface temperature is lower than Ms and the cooling rate of core region is higher than 0.05 ?/s(the critical cooling rate of pearlite transformation).In addition,the nozzle and riser differential thermal cooling quenching method is proposed to solve the uneven section hardness caused by macrosegregation.Finally,it is found that the trend of toughness and strength is inversely proportional after in-depth study of 718H steel tempered from 5000C to 650?.The evolution of mechanical properties during tempering depends on precipitates morphology,dislocation density and grain orientation.(2)The optimal high temperature diffusion process was determined based on the calculation of alloying elements diffusion model and the characterization of microsegregation experiments.Simultaneously,the mechanism of microsegregation on the fracture of 718H mold steel matrix was deeply studied.The results indicate that the optimal process is determined to be 1250?+12 h by accurate statistical analysis of grain size,microhardness difference and segregation ratio of Cr and Mo elements.The optimal process improves the hardness uniformity by largely eliminating the microsegregation of the elements.At the same time,the band structure after forging is reduced,which improves the transverse impact property and isotropic of mold steel tempered at 540?-700?.In addition,the yield strength is improved tempered at 540 ??650?,but it is slightly lower than that of the untreated sample tempered at 700?,which can be attributed to the synergistic effect of precipitation strengthening and grain boundary strengthening mechanism.(3)The optimal addition amount and method of high purity RE(Ce and La)alloys were determined by thermodynamic calculation and experimental characterization of deep deoxidization,and by comparing the comprehensive mechanical properties of materials.The results indicate that the total oxygen(T.O)content can be reduced from 15 ppm to 6 ppm by adding 0.022 wt.%RE when the T.O content in steel is less than 20 ppm.The strip inclusions are modified into submicron ellipsoidal RE-inclusions by adding 0.012 wt.%and 0.022 wt.%RE.Meanwhile,the proportion of coarse inclusions with the diameter exceeding 10 ?m obviously reduces by 11.5%,while the proportion of small inclusions with the diameter less than 2 ?m increases by 34%and 41.8%,respectively.The comprehensive advantages of deep deoxidation and modification of inclusions by RE have a significant effect on improving the impact energy and isotropic properties.In addition,soluble RE atoms are prone to defects such as grain boundaries and phase boundaries,which reduces the grain boundary energy,blocks the diffusion channels of C atoms,and improves the stability of undercooled austenite.However,when the RE content increases to 0.07 wt.%,the impact energy,tensile strength and high temperature fatigue crack propagation resistance are reduced to varying degrees due to the large number of large-sized inclusions.Considering the evolution of inclusions and mechanical properties,the optimum RE content is determined to be 0.012 wt.%to 0.022 wt.%.(4)A novel HSTPM plastic mold steel with high hardenability and comprehensive properties was developed by reducing C and Cr,increasing Mo,V and RE elements.The results indicate that HSTPM steel exhibits higher strength and hardness as well as toughness compared with imported high-end 718H steel tempered from 530? to 650?.The achievement of the strength and hardness is mainly attributed to the addition of Mo and V elements to increase the nano precipitation strengthening and dislocation strengthening of matrix.In addition,the purification of molten steel and modification of inclusions by RE treatment as well as the lower C and Cr contents of HSTPM steel are mainly responsible for the improvement of impact toughness.(5)The large cross-section pre-hardened 718H-RE plastic mold steel was trial-produced through the research of materials and development of key technologies.The results indicate that a series of common technologies have been successfully applied to the actual production process of 718H-RE plastic mold steel with a thickness of 835 mm,such as purification smelting process by RE,high-temperature diffusion annealing process,normalizing process,water-alternating quenching and combined with nozzle and riser differential thermal cooling quenching process and precise tempering temperature control.The T.O content of the module is only 6 ppm,which reaches the T.O level(6 ppm?10 ppm)of imported high-end 718H mold steel.Simultaneously,the tiny inclusions are evenly dispersed in the module.Therefore,718H-RE steel meets the purity requirements of mold steel in the National Key R&D Program of the 13th Five-Year Plan.In addition,the overall hardness of the module is controlled at 33 HRC-37 HRC,and the difference of the section hardness is stable within ±1.1 HRC after modulation treatment,which reflects the high quality of large cross-section 718H-RE mold steel. |
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