| Hot press forming of 22MnB5 steel has been widely used for fabrication of the automobile structural parts of ultra high strength steels to meet the requirement of reducing automobile weight for energy saving and improving safety.Due to its high strength properties and good formability,hot stamping steel has been applied more and more widely in the automobile industry.However,for traditional uncoated hot forming steel,one of the biggest problems in the process of heating forming is the steel plate surface decarburization and oxidation of the steel surface.The decarburization will reduce the strength of the steel surface.And the oxidation of the steel surface will lead to lower die life and substantially reduce the production efficiency.In order to avoid surface decarburization and oxidation,in recent years,hot-dip galvanized(GI)steel plate has not only solved these problems,but also possessed good corrosion protection and cathodic protection.In this paper,the galvanisability of 22MnB5 steel in hot-dip galvanizing process and the microstructure change of coating during heating were studied.In addition,the effection of microstructure and property at different heating process and the crack generation at different hot stamping process also were studied.These results have important guiding significance to the development of coated hot stamping steel.The study found that oxygen-potential is the key influence factors of galvanisability of 22MnB5 steel.High oxygen-potential could make Mn oxidized inside of the steel which makes oxide be reduced on the steel surface.Low oxygen-potential can reduce the presence of on the surface of steel,as not enough oxygen gets to the surface.Dew point has the most impact in oxide,followed by temperature and H2 component.It would be best for the galvanisability of 22MnB5 steel when both of dew point and temperature was too low.The steel surface can make the inhibition layer particles of hot-dip galvanizing plate become more continuous and compact and can improve the quality of Zn coating.H2 component can also affect the galvanisability of 22MnB5 steel,but the effect was not obvious.When the ratio of H2 is high,due to the strong reducing capacity of the atmosphere,the surface has less oxide and good galvanizing effect.According to the experimental results,it can be seen that in the heating process,the GI coating changes from the initial η phase to theζ,δ,Γ1,Γ,α-Fe(Zn)phases as the temperature increase.After heated at 900℃,the coating was almost α-Fe(Zn),only a small amount of Γphase and ZnO presented in the surface.The Al in the coating migrate from the interface to the surface to form a continuous sequence of Al2O3.The increase of the temperature and the prolongation of the holding time make the coating become more of the phase of α-Fe(Zn),and the thickness of the coating becomes thicker.However,when the temperature is higher than 930℃,the holding time is more than 8min,the surface of the coating will reduce the Al2O3,ZnO increased.According to the study of crack in hot stamping process,we can seen that the main reason for the occurrence of macro-cracks is the liquid metal embrittlement and micro-crack is mainly caused by the friction between the die and the steel plate during the press hardening.Macro-cracks extend to the matrix,while micro-cracks only propagate in the coating,with micro-cracks not affecting the application.The increase of the heating temperature and the prolongation of the holding time will reduce the macro-cracks,while the pressing temperature is the main factor to control the macro-cracks.When the pressing temperature is less than 782℃,there are few macro-cracks and few side cracks.When the pressing temperature is higher than 782℃,the macro-cracks increase significantly,and a little of the side micro-cracks become macro-cracks. |
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