Control Technology Of Microstructure And Precipitation In Unidirectional Solidification Of Hot-Work Die Steel

With the continuous progress of modern industrialization,the processing and manufacturing industry in rapid prototyping fields,in terms of die-casting,hot-forging and hot-extruding concerning light metal(such as alloys of alumina and copper),have been developed rapidly.However,the service life of hot-work dies used in high-temperature environments is the main factor to limit its development.Therefore,the research and development of a newly hot-work die steel,which has a long service life,high strength and high toughness used in these service conditions with elevated temperature,is a key issue to be solved.Hot-work die steels with martensitic matrix are widely used all over the world.However,these martensitic hot-work die steels will drastically deteriorate when the service temperature of mold-cavity higher than 650?,and these steels are difficult to applied in high temperature service condition.Therefore,based on this point,an austenitic hot-work die steels containing high-Mn concentration was selected to be investigated to apply in elevated temperature.The microstructure and precipitated carbides of austenitic hot-work die steel were analyzed by thermodynamic software(Thermo-Calc).In order to reduce the central defects in ingot remelted through ESR(conventional electroslag remelting),ESR-CDS(continuous directional solidification of electroslag remelting)was used to improve the solidification microstructure,and optimize the solidification model.The experimental results showed that the mushy zone between central equiaxed crystal grains and columnar crystal grains could be alleviated through ESR-CDS,and obtained entirely columnar crystal grains grown paralleled to the axis of ingot.In addition,the segregation of elements in ingot was alleviated and the dendrite arm spacings in ingot were reduced,as well as the growth pattern of dendrites was optimized.The characteristics of carbides precipitated in ingot and the powder of carbides extracted from ingot through electrolytic extraction were analyzed by SEM-EDS(scanning electron microscope equipped with energy-dispersive spectroscopy).The effect of electroslag process parameters on the mechanism of forming carbides.The results indicated that the amount of primary carbides could be decreased,and the sizes of primary carbides could be reduced,as well as the morphologies of primary carbides could be refined through ESR-CDS.Moreover,the morphologies of MC type carbides were changed from short rod-like shape in ESR to lamellar-like shape in influenced from ESR-CDS.However,the morphologies of M2C type carbides in ingots remelted through ESR and ESR-CDS were similar.In addition,the growth patterns of MC carbides during solidification were like dendritic morphologies.A thermodynamic study,a classical solidification model and a growth model for MnS inclusion were used orderly to interpret the metallurgical results.And the results showed that the number density and the size of inclusions in ingot were much more reduced through ESR-CDS process in comparison with ESR process.The starting time for the precipitation of MnS inclusion was retarded,then the local time for the growth of MnS inclusion was reduced.The removal mechanism of MnSinclusion through ESR-CDS was analyzed by the number simulation of solidification in ESR-CDS about the motion track of inclusion.To improve the high temperature properties of hot-work die steels,the rare earth alloying method in austenitic hot-work die steel was used to optimize the solidification microstructure and primary carbides in ingot.The results showed that the austenite grains could be refined through rare earth alloying method,as well as the intragranular twin boundaries could be increased resulted in the increasing in grain boundaries area ratio.Therefore,the strength and toughness of austenitic hot-work die steel could be simultaneous improved due to the refined grains and impeded movement dislocation.