| Hot Stamping of Ultra-High strength steel is an efficient solution to realize the weight reduction and safety improvement of automotive.Zinc based coatings of press hardened steel has gradually developed due to providing good cathodic protection,however,the cracks during hot stamping was the mainly problem,which limited the use of zinc based coating of press hardening steel.In this paper,cracks generation and propagation mechanism under different deformation method were studied by OM?SEM?EDS?GDOES and EPMA,It was found that: when the hot forming was at high temperature,Liquid metal induced embrittlement cracks were easy to occured,coating mainly consists of solid ?-Fe(Zn)phase and liquid zinc,cracks were generated at the interface between ?-Fe(Zn)and liquid zinc,the liquid zinc was penetrated along the austenite grain boundaries and reacted with Fe to generate the ?-Fe(Zn),cracks were continuously propagated along the ?-Fe(Zn)/liquid Zinc and finally caused the base metal breakage.when the hot forming temperature was at low temperature,coating was consist of solid ?-Fe(Zn)and ? phase,cracks were initiated at the interface between ?-Fe(Zn)and ? phase,and propagated along ?-Fe(Zn)grain boundaries embrittled by liquid zinc during austenite process to a thin ferrite layer containing Zn between the coating and base metal,which does not extend to the martensite matrix,and occurred the micro-cracks inside the steel,the length of micro-cracks was determinated by the thickness of thin Ferrite layer.To avoid or reduce the liquid metal induced embrittlement cracks,one way was controlled the coating phase with mainly solid ?-Fe(Zn)and liquid phase far away from the base metal during austenite process prior the hot stamping,another way was to hot forming at low temperature under 782°C which was the melting point of ? phase.The effect of heating temperature,soaking time and initial hot forming temperature on coating microstructure,composition,cracks and mechanical properties of base metal were investigated in detail.With the heating temperature or soaking time increased,the content of ?-Fe(Zn)phase in the coating gradually increased,the content of ? phase in the coating gradually reduced and was far away from base metal.When the heating temperature 880~920°C with soaking 5~7min and initial forming temperature at 680~750°C,cracks were only occurred at the coating and not propagated into the substrate,however,the tensile strength would decreased at a lower hot forming temperature.The effect of pre-alloying process on coating microstructure,composition and cracks were investigated,the coating consists of almost solid ?-Fe(Zn)phase after pre-alloyed at 550°C for 5-10 min and then heated at 900°C for 5min,which can avoid the risk of Liquid-metal-induced-embrittlement cracks during hot stamping process.The presence of Zn in the coating could provide the cathodic protection.The influence of hot stamping parameters on the friction coefficient and induced strain on austenite continuous cooling curves were analyzed to determinate the process parameters of hot stamping industrial trial,B-Pillars were manufactured in the industrial line by direct hot stamping method,mechanical properties and dimension accuracy of B-pillar meet the requirement of car manufacturer,cracks were only occurred at the coating of component,?-Fe(Zn)phase exiting in the coating had the cathodic protection and provided a good corrosion protection to the substrate. |
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