The Study On Microstructure And High Temperature Friction And Wear Mechanism Of New Type Hot Stamping Die Steel

A new type of hot stamping die steel with high thermal conductivity and high wear resistance was developed for the development requirement of hot stamping technology.The microstructure and performance characteristics were systematically investigated by means of scanning electron microscopy(SEM),transmission electron microscope(TEM),UMT-3 type high temperature friction and wear test system and white-light interferometer testing methods and so on.The precipitation and evolvement rules of carbides of the new type hot stamping die steel during tempering process were explained.The high temperature friction and wear mechanism and the influence factors of the new type of die steel were expounded.The high wear resistance was investigated by contrasting tested with other high performance hot work die steel at simulate service condition of hot stamping process.A numerical model of a B-pillar component was established to investigate the effects of the thermal conductivity of the die material on the press quenching stage of the die during the hot-stamping process.The main results are as follows:(1)The hot stamping die steel is confirmed through the investigation of microstructure characteristics and heat treatment process of new type hot work die steel.The optimum heat treatment process is austenitizing and quenching at 1080? then following tempering at 640? twice for 2h.The hardness is 53 HRC,the average impact energy is 182 J and the thermal conductivity is 46W/m·k after heat treatment process.The new type hot stamping die steel with high temper resistance,which hardness is about 46 HRC after tempered at 620? for 20 h.Its hardness higher than the H13,CR7 V,HTCS-130 and DIEVAR steel about 17HRC?13HRC?10HRC ?7HRC under the same experiment condition,respectively.(2)There is a mass of secondary carbides precipitation during temper process in the new type hot stamping die steel.The M2 C type carbides precipitated in the matrix when tempered at 560?.When the temperature rise up to 600?,the VC carbide is start precipitation.The M2 C carbide is transformed into M6 C carbide on the condition of temper temperature up to 620?.When the temperature is 660?,the carbides are M6 C and VC carbides in new type hot stamping steel.(3)The results show that the new type hot stamping die steel has lower friction coefficient and greater wear resistance compared with H13 steel under test conditions.This owes to its high oxidizability and high temper stability.The high oxidizability of the new type hot stamping steel facilitates the generation and growth of the tribo-oxide layer,which is maintained at high temperature because of the high temper stability of the steel.The new type hot stamping die steel,which exhibits high temper stability,maintains a certain strength and hardness,avoiding large plastic deformation and effectively supporting the tribo-oxide layer at high temperature.These features postpone the transition from mild to severe wear and ensure that conditions of mild oxidative wear are maintained.Mild oxidative wear is the dominant wear mechanism for the new type hot stamping die steel between 400 and 700?.(4)The carbides layer which under tribo-oxide layer may formed via the aggregation of carbides during high temperature friction process.It improves the support provided to the tribo-oxide layer and impedes the growth of oxides into the matrix and makes sure the tribo-oxides layer with higher thickness and under perfect condition.There is not mass of cracks in the tribo-oxide layer and matrix.The tribo-oxides peel off from the layer surface with tiny debris on the new type hot stamping die steel wear surface.Therefore,the new hot stamping die steel shows low wear rate.(5)The temper temperature has a significant influence on the wear resistance of die steel.The new hot stamping die steel has favorable wear resistance after tempered at 640?.The hardness and impact energy influence the wear resistance of die steel together.The high impact energy is benefit to improve the wear resistance of die steel when the hardness similarity.The wear resistance decreased with the increase of temperature under experimental condition.The new hot stamping die steel has better wear resistance then the other die steels under same experiment condition in this study.The high wear resistance of the new hot stamping die steel derives from the high hardness and toughness combination and high carbides volume percentage after heat treatment.(6)The results show that the thermal conductivity of the die material has a more significant influence on the quenching stage than the forming stage.The thermal conductivity of the new type hot stamping die steel is higher than H13 steel by 76.23% at 25-300?,and the maximum cooling rate of the component increases by 48.49% and consequently improves the quenching efficiency of the hot-stamping process by 31.82%.As the thermal conductivity of the die steel increases,the maximum temperature of the die decreases and its temperature uniformity improves.This is benefit to the uniformity of die temperature and then prolongs the serving life of the die.Moreover,to improve the efficiency of the hot-stamping process,steels that possess high thermal conductivity at low temperature ranges are favorable.