The Microstructure And Mechanical Properties Of A New Type Of Ultra-high Strength Steel For Automobile B-pillar By Hot Stamping Forming Process

B-pillar is one of the most important part for car safety.The B-pillar of a factory was taken as the research object by using finite element method to analyze the hot stamping forming process of the 22MnB5 steel car B-pillar.The most difficult deformation zone of the car B-pillar during forming process was selected to reasonably design and manufacture the mold.Then,a new type of ultra-high strength alloy steel was designed was produced to plate by rolling process for the following hot stamping forming experiment.Through finite element analysis,A large temperature gradient was found in the blank holder area caused by the close contact of cold blank holder and plate,which induced strength in the blank holder area was much higher than that in the drawing depth.During the deformation process,the sheet material in the blank holder area is difficult to enter the fillet,which leads to the sheet material broken.The blank bead contacted with the sheet material point has little effect on the temperature field.During the deformation process,the blanks area of the blank holder can smoothly enter the round corners.Therefore,the using gap blank holders was more reasonable for the hot stamping forming.It was found that the wrinkles always appear in the hard deformation zone of B-pillars for the conventional hot stamping process by finite element analysis,and then the reason caused wrinkles was analyzed.The effective way for solving this problem was found by pre-forming the material in the region,in which the internal clearance of blanking ring can effectively eliminates the wrinkles during hot stamping process.Finally,the optimum hot forming process parameters of the car B-pillar were determined as the initial heating temperature of the sheet material of 930°C and the stamping speed of 150 mm/s.The designed ultra-high strength alloy steel was rolled to obtain 1.5mm thickness sheet.After air cooling at 880°C and tempering at 260°C,the tensile strength,yield strength and hardness of the material reaches 1600 MPa,1298MPa and 47 HRC,respectively.The hot forming molds was designed and manufactured for car B-pillars to solve the problem of hard deformation region in car B-pillars,in which internal pressure plate was added in the mold to eliminate wrinkles generated during the hot forming process.And then,hot forming of rolled ultra-high-strength alloy steel plate was performed to obtain the experimental specimens in which there was less wrinkles appearing that proofs the above analysis and mold design.After hot stamping,the tensile strength,yield strength and the hardness of the experimental piece can reach 1213 MPa,986 MPa and 37.9 HRC,respectively.