Microstructures And Properties Of New-style Hot Stamping Die Steel 4Cr4Mo2NiMnSiV

With the development of hot-work die steel at home and abroad,the working temperature and pressure of the die are being continually improved,higher requirements have been put forward for the properties of the steel,especially thermal stability and tensile properties under the condition of high temperature.4Cr4Mo2NiMnSiV steel is a new type of hot-work die steel designed on basis of its service environment and the alloying principle.Thermo-Calc software and Jmatpro software were applied to obtain equilibrium phase diagrams and CCT curves for predicting microstructures and properties of the material in the nonequilibrium.The critical phase transition points of 4Cr4Mo2Ni MnSi V steel were measured,at the same time,its as-cast microstructure and hardness were studied,the results show that:（1）When the heating rate is 10℃/min,the critical phase transition points are Ac₁=809℃,Ac₃=884℃;When the heating rate is 20℃/min,the critical phase transition points are Ac₁=825℃,Ac₃=904℃.（2）The hardness of the as-cast material is 53.8 HRC,its casting-state structure is mainly martensite containing a small amount of carbide precipitation;As-cast organization exists serious composition segregation,which are mainly alloying element segregation of C,Mo,Cr and V,seen from energy spectrum analysis.Based on the critical phase transition temperatures and heat treatment principles of alloy steels,a series of heat treatment process were designed by hardness testing,metallographic observation,SEM observation,XRD analysis and energy spectrum analysis,and combining room temperature impact tensile tests,microstructure and properties of the steel were studied,the results show:（1）annealing pretreatment can improve the as-cast segregation,but cannot eliminate.After“730℃×5h”annealing pretreatment,material structure and hardness have a better improvement,the microstructure is spheroidal pearlite and its hardness is about 24HRC.（2）At 900℃¹080℃oil quenching,the hardness shows a trend of rise first then fall with the increasing of quenching temperature.At 1000℃quenching,material hardness peak58.2 HRC,the microstructure is mainly lath martensite with a small amount of carbides.（3）After 350℃⁶50℃secondary tempering,material appeared obvious secondary hardening phenomenon near 500℃;Temper brittleness occurs between 500℃⁶00℃,impact energy appears the minimum near530℃,which is related to the strip-shaped segregation band and the segregation of carbon and impurities such as S,Si and Mn elements in the grain boundary in tempering process.Room temperature tensile strength shows a trend of rise first then fall with the tempering temperature,it achieves the highest strength and hardness,1775MPa and 59.6 HRC respectively at 500℃;Tensile break elongation and energy at tensile strength also rise first then fall,it peaks 7.84%and 102.7J at 590℃;Tensile fracture appears quasi cleavage fracture characteristics,whose fracture surface has many torn edges and a small amount of dimples.（4）It gets the optimal heat treatment process of 4Cr4Mo2NiMnSiV steel:"730℃×5h"pretreatment+1000℃oil quenching+590℃second tempering.At the same time,tempering stability,thermal stability and high temperature tensile properties of the steel was emphasized to study,the results showed that:the material at1000℃oil quenching has a better tempering stability comparing 960℃and 1040℃oil quenching;at 620℃⁷00℃,4Cr4Mo2NiMnSiV steel has a better thermal stability、high temperature tensile strength、high heat resisting property than H13 steel,at the same time it has much high temperature tensile plastic toughness,high temperature tensile fracture shows typical dimple fracture.