Research On Hot Stamping Process Of Zn-coated High Strength Steel

High strength steel hot stamping parts have been widely used in automotive industry because of their ultra-high strength（more than 1500 MPa）.To avoid the oxidation and decarbonization of traditional high strength steel during hot stamping process,the Zn-coated high strength steel was applied,which can also enhance the corrosion resistance of hot stamping parts.However,due to the low melting point of zinc,the Zn coating will change into liquid state under high temperature.With traditional hot stamping process,the liquid metal induced embrittlement（LMIE）will occur in Zn-coated steel,which will lead to the substrate cracking.In this paper,the hot stamping process of Zn-coated high strength steel is researched,and experiments are implemented under different process parameters,finally,a suitable hot stamping process of Zn-coated high strength steel is obtained.The main work is as follows:（1）The effect of heating temperature,holding time and deformation temperature were studied,which play a key role in hot stamping process of Zn-coated high strength steel.Firstly,to determine the appropriate range of the process parameters,the flat die experiments were implemented,according to the surface quality of the coating and the mechanical properties,the effect of heating temperature and holding time is investigated,and an appropriate heating temperature is between 860℃⁸80℃,heating time is between 3⁶ minutes.Then,the thermal simulation tensile test is implemented,and the stress-strain curves under different conditions were compared to figure out the effect of the three process parameters.and the X-ray diffraction（XRD）analysis and scanning electron microscope（SEM）observation were carried out on the tensile specimens to analyze the phase composition and micro-morphology.The results show that the heating temperature has little influence on the substrate cracking,but the holding time has great influence.When the holding time exceeds 130 s（heating time exceeds 3 min）,the substrate cracking will not occur in direct forming;when the holding time does not exceed 130 s,the substrate cracking will occur in direct forming,but it can be prevented by lowering the deformation temperature to a certain temperature（700℃）.（2）The results of the previous chapter show that the heating temperature has little effect on the substrate cracking.When the heating temperature is 880℃ and the heating time is more than 3 minutes,the parts can be formed directly,when the heating time is less than 3 minutes,the substrate cracking can be avoided by reducing the deformation temperature.According to the results,two process routes are concluded:"long-term heating,direct forming"and"short-term heating,pre-cooling then forming".In this chapter,the two process routes are verified by the part experiments of hot stamping anti-collision beam.Firstly,“long-term heating,direct forming”,the sheet is transferred to the die directly after long heating time.The results show that the LMIE will not occur when the heating time is longer than 3 minutes,when the heating time is less than 3 minutes,the coating will be damaged seriously in the large deformation area and the substrate also emerges cracking.Moreover,short heating time will lead to inadequate austenization,resulting in the bad performance of hot stamping parts.Secondly,“short-term heating,pre-cooling then forming”,after short heating time,the sheet is pre-cooled to a certain temperature before forming.The results show that when the deformation is low（700℃）,there is no cracking emerges in the substrate,when the deformation temperature is high（800℃）,micro-cracking emerges in the substrate.In addition,when the deformation temperature is low,the cooling rate in the pre-cooling stage should be as fast as possible to avoid the insufficient martensite transformation.（3）The corrosion behavior of Zn-coated hot stamping part was studied by the neutral salt spray test,and its corrosion resistance was evaluated by comparing with the Zn coated steel and uncoated steel.It was found that the corrosion resistance of uncoated steel is very poor,and it suffered serious corrosion in short time.The coating of Zn-coated steel consists ofδphase,which can provide strong cathodic protection with high zinc content,but it can not resist corrosion for a long time because of the fast consumption of coating.The main corrosion products are Zn₅（OH）₆（CO₃）₂ and Zn₅（OH）₈Cl₂·H₂O.As for the Zn-coated hot stamping part,the thickness of the coating changes from 10μm to 20μm after hot stamping process.The content of Fe increases in the coating,and the main phase consists ofα-Fe andΓphase.The corrosion resistance of Zn-coated hot stamping part is not as good as Zn-coated steel in a short time,but the consumption of coating is slower and the thickness of coating is thicker,so it can resist corrosion for a longer time.The main corrosion products are Zn₅（OH）₈Cl₂·H₂O.In this paper,a hot stamping process suitable for Zn-coated high strength steel is proposed,and the in-door anti-collision beam is manufactured,which verifies the feasibility of the process.