| Hot blanking is an integrated process that heating the high-strength steel to the austenitic state and then blanking and quenching in die.Hot blanking technology can significantly improve the forming properties of high-strength steel and mechanical properties of parts,hence,hot blanking can produce various precision blanked parts with comprehensive mechanical properties,such as truss plates,thin gears,thin chain wheels,racks and certain other plate parts for transmission.These parts are generally formed using the fine blanking process;then,the parts requiring wear resistance are carburized and quenched or induction hardening and other heating treatment for improving the hardness and wear resistance of the transmission zone.Heating treatment would lead to the shape or size variation of parts,and affecting the shape and dimensional accuracy.The hot blanking process can replace traditional process that fine blanking firstly then heating treatment,and produce the parts with high hardness and strength,avoiding distortion of the parts during the heating treatment.To obtain the hot blanked parts with high quality,some scholars have researched the warm and hot blanking of high-strength steel.The studies about the blanking process presently aimed at solving the difficulty appearance in the piecing and trimming for high-strength.The research results that using the hot blanking to produce the precision blanked parts are few,and the study on the finite element model and numerical simulation of hot blanking are also rare.To explore the optimal hot blanking process of boron steel and obtain the optimal technological parameters.To investigate microstructural transf-ormation,dimensional accuracy variation,fracture mechanism at elevated temperature,and fracture surface microscopic morphology of hot blanked parts,evaluate the effects of blanking temperature and die clearance ratio on fracture quality,shape and dimensional accuracy,fracture angle,and the maximum blanking force.A series of hot blanking experiments for the B1500HS steels were executed at various blanking temperatures(450 0C,500℃,550℃,600℃,650℃,700℃,750℃ and 800℃)and die clearance ratios(25%,20%,18%,16%,14%,12%,10%,and 8%),using the blanking die with special design structure.According to the cooling curves measured in the hot blanking experiments and continuous cooling transformation(CCT)diagrams of B1500HS steel,the effects of blanking temperature on microstructural transformation and mechanical properties of the parts were analyzed.The blanking temperature and die clearance ratio effects on the quality of the sheared section,the dimensional accuracy,and fracture angle were analyzed by using the Zeta-20 three-dimensional surface topography profilometer.Based on the load curves obtained at various blanking temperatures and die clearances ratios,the blanking temperature and die clearance ratio effects on the maximum force were analyzed.The surface morphology of the sheared section for the blanked workpieces was observed and analyzed by using the Nova Nano SEM 450 high resolution scanning electron microscope(SEM)and the JXA-8230 electron probe microanalysis(EPMA).The experimental results demonstrate that with decreasing die clearance ratio,the diameter of the blanking parts increases at the same blanking temperature.When the die clearance ratio remains constant with increasing blanking temperature,the dimension deviation of the parts has a fluctuation tendency of "negative growth—positive growth—negative growth." The blanking parts have a higher accuracy as the blanking temperature is in the range of 450～500℃,600～650℃,or 750～800℃.While increasing the blanking temperature,the micro-hardness of the parts increases.The microstructure of the parts is full martensite and the micro-hardness is about HV 550 as the blanking temperature is in the range of 650～800 0C.Furthermore,increasing the blanking temperature,the width of burnish zone increases,the width of fracture zone decreases,fracture angle decreases,the maximum blanking force decreases.The sheared sections of the parts demonstrate ductile fracture mainly,accompanied by local brittle fracture as the blanking temperature range is 450～600℃,whereas the ductile fracture has a high amount of fine equiaxed dimples as the blanking temperature range is 650～800℃.Parts with a high dimensional accuracy,better fracture perpendicularity,wide burnish zone,better mechanical properties,and wear resistance can be obtained at blanking temperatures 750～800℃.The investigation on the distribution and variation of the stress state in sheared zone at elevated temperature are the basis of the hot blanking mechanism,the fracture criterion and fracture shreshold are selected reasonably and predicted exactly are crucial for simulating the fracture in hot blanking.To further explore the fracture mechanism of boron steel in hot blanking process,evaluated the stress and strain distribution in sheared zone during hot blanking,and research the effects of hydrostatic stress,equivalent stress,equivalent strain,and material flow on the location the crack occurs.The finite element model of hot blanking at elevated temperature was established using Deform v11.0,and the fracture shreshold with different blanking temperature and different fracture criterion(Normalized Cockroft&Latham fracture criterion,Brozzo fracture criterion,Freudenthal fracture criterion,Oyane fracture criterion and Rice&Traccy fracture criterion)were predicted accurately using the method of "regression fitting—calculation—prediction—correction".The numerical simulation for the hot blanking at elevated temperature was carried using the calculated fracture shreshold,and the fracture position of parts during hot blanking and fracture profile were obtained.By comparing the simulation results and experimental results,it is indicated that the predicting results based on Oyane fracture criterion and Brozzo fracture criterion are in good agreement with experimental results.The fracture occurs at the die and punch edge as the hydrostatic stress at cutting edge reaches the maximum value.The fracture shresholds calculated based on the method of "regression fitting—calculation—prediction—correction"have better reliability and practicability.The theoretical calculating and testing results of the forming limit curves and fracture curves for steel sheet can be used to predict the fracture thresholds for the corresponding fracture criterion,which provides an efficient and convenient way for obtaining fracture threshold.In addition,the hot forming limits of boron steel at elevated temperature provides a theoretical basis and data reference for forming performance evaluation and hot stamping testing using finite element technology.As the boron steel B1500HS deformed at elevated temperature,in the complete austenitic state,it accord to 0-angle necking type whatever the strain state is in left or right hand of forming limit diagram,which is submit to the M-K simplify model.However,the constitutive equation affects significantly the prediction of the forming limit diagram(FLD)of the sheet metal.In this paper,the modified Arrhenius model including the deformation activation energy and the deformation temperature,describing the hot deformation of the sample with austenitic microstructure,is used to predict the forming limits of boron steel B1500HS,combined with the Logan-Hosford yield criterion and the Von Mises yield criterion.The theoretical calculation of the forming limits for B1500HS steel at elevated temperature and for different conditions(including different forming temperatures of 600～900℃,different mean strain rates of 0.1～10/s,different initial inhomogeneities f0 of 0.997～0.999)was carried out,and the FLD predicted by the theoretical calculation is in good agreement with the experiment results.It is concluded that with decreasing the initial inhomogeneity of the material,the hot forming limit curves for B1500HS steel at elevated temperature decrease,and the initial inhomogeneity coefficient f0=0.997 is better for predicting accurately the hot forming limits of boron steel.With increasing forming temperature,the forming limit curves,calculated for B1500HS steel at elevated temperature,increase.With increasing mean strain rate,the forming limit curves of B1500HS steel at elevated temperature increase slightly. |
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