Investigation On Galling Mechanism With Bending Forming Of High Strength Steel Sheet For Automobile

Automobile lightweight has been one of the topics for the sustainable development of automobile in the future due to the requirements of green manufacturing such as energy conservation,environmental protection.High strength steel has been the most widely used material in the car for its superior performances such as high strength,low density,collision energy absorption and so on.However,with the increased strength of high strength steel,the galling behavior including scratch,adhesion,pit occurr on the surface of die and sheet during the forming process,which decrease the service life of die and make surface quality of formed parts worse.Thermal diffusion carbide coating process is a promising surface treatment technology to improve wear resistance and therefore widely applied in auto industry.The galling mechanism of Cr12 steel and VC coating Cr12 steel coated by TD method is studied systematically by the U bending forming test of DP780 high strength steel sheets.Based on the FEM-Archard wear model,the galling behavior is quantificationally evaluated during the forming process of DP780 high strength steel sheets.The results show that slight glling occur on the surface of Cr12 steel when the forming number N=3.As the forming number comes to the N=33,severe galling with a large amount of tumors is detected which means the galling mechanism is adhesive wear.For the VC coating steel,slight micro glling is observed at initial stage(N=30)and the critical Ry value that severe galling occur is 1.554μm;the region where severe galling firstly appear is 60-70° on the die radius and then extend to its both side;Ploughing and scatch are the mian mechanism of surface damage at initial stage and severe scratched area are 0-10° and 30-50° of die radius.Then,scratches and transverse cracks on the DP780 sheet surface and tumors on the die surface indicate the sheet material have transferred to the die surface,which refer to galling,and a transition to adhesive wear take place at this stage.With the forming test proceeded(N=660),abrasive wear is of the main mechanism due to the appearance of wide deep grooves and pits on both sheet and die surface.Compared with Cr12 steel,VC-coated Cr12 steel treated by TD process expresses more excellent anti-galling performance during sheet metal forming process.Coupling the Archard wear model,the revolution of galling behavior is predictedat different processing parameters by Abaqus during the forming of DP780 high strength steel sheets.The results show that the severe galling regions predicted by Archard wear model have a good coincidence with experimental results.The profile of die radius have an effect on the galling behavior of die surface and deviant elliptic arc have better anti-galling performance of sheet metal forming than standard arc,deviation and elliptic arc.