Basic Theory And Experimental Investigations On Selective Heating And Press Hardening Of High Strength Boron Steel Plate

The automobile manufacturing industry is an important pillar industry of our national economy.Improving the safety performance and energy saving and emission reduction ability of automobiles goes with the green development requirements of"Made in China2025"plan.It has become one of the main driving forces for the innovation and development of the automotive industry.The hot stamping high-strength steel and hot stamping technology meet the requirements of the automobile industry for high strength and reducing weight at once,which have been paid more and more attention and application.However,although the high strength steel pressure hardened parts have high strength and hardness,their energy absorption is limited and their comprehensive mechanical properties are not high.Therefore,the design of high strength steel press hardening parts with gradient performance distribution according to the load distribution form has become the goal of automotive anti-collision core parts.It is of great significance to improve automotive collision safety and promote automotive lightweight,and has the potential to realize functional customization of automotive parts.In this paper,22MnB5 is taken as the research object,aiming at the selective heating and press hardening,the constitutive relationship of multi-phase zonal materials,the transformation process of micro-structure and the kinetics of phase transformation of boron steel sheet are systematically studied.The feasibility and controllability of the process were verified by numerical simulation and local induction heating and press hardening experiments.The gradient performance parts were obtained and its re-deformation properties were evaluated by experimental.The main research contents and contributions of this paper include the following points:(1)The phenomenon of dynamic strain aging of high strength boron steel was studied and the reasonable heating temperature of selective heating was obtained.Based on the distribution of initial non-uniform temperature field,the rheological stress characteristics of high temperature austenite zone,phase transformation transition zone and original structure zone have been studied respectively.Based on Arrhenius model,the constitutive relation austenite phase zone and original phase zone of boron steel were established.By improving Arrhenius model,the constitutive model of transition multiphase zone was established by characterizing the deformation activation energy in the transition zone as a function of temperaturethe.(2)The microstructural evolution of high strength boron steel during press hardening was systematically studied by means of in situ observation experiments.The precipitation of carbides and the effect of alloy phase B during austenitizing were studied.The higher the heating temperature is,the more obvious the carbide segregation is.The segregation and solution of B affect the austenite grain uniformity.The phase transformation mechanism and the effect of cooling rate of boron steel during cooling were studied.At low cooling rate,upper bainite transformation occurs and the hardness is 194HV;at medium cooling rate,upper bainite and lower bainite transformation occur and the hardness is 289 HV.Increasing the cooling rate can reduce the starting temperature of bainite transformation and accelerate the growth rate.At high cooling rate,only martensite transformation occurs and the hardness is 426 HV.The starting temperature of martensite transformation decreases with the decrease of austenite grain size.When the grain size is small than 30?m,the decreasing speed of M_s point is accelerated.(3)Based on the improved JMA model,the activation energy and JMA parameters of austenite transformation of experimental steels were calculated,and the non-isothermal austenite transformation kinetics model of boron steel was established.Based on K-M model,a cubic polynomial relationship between martensitic transformation kinetics coefficient and temperature was found,and a martensitic transformation kinetics model of boron steel was established.The effect of the austenite phase deformation on martensitic transformation of boron steel was studied.The martensite transformation is sensitive to the strain of austenite phase deformation,but not to the strain rate.The strain increases by20%,and the retained austenite increases by 10%after rapid cooling of experimental steel.(4)The finite element simulation model of selective heating and press hardening was established,and the basic laws and characteristics of temperature field and stress-strain field of sheet metal under this process are revealed.The influence of initial heating temperature gradient and stamping speed on pressure hardening process is analyzed.The results show that the average cooling rate in the high temperature zone is 65-80℃/s,which is higher than the critical cooling rate of martensitic transformation,and the forming temperature in the low temperature zone is above 500℃,which is higher than the dynamic strain aging temperature.Reducing the initial heating temperature gradient and increasing the stamping speed can effectively reduce the stress level of sheet metal and reduce the residual stress in the forming process.(5)A local induction heating and press hardening experimental platform was built and the microstructure of gradient distribution parts are martensite phase,martensite,ferrite and retained austenite phase,ferrite and pearlite primary phase consecutively,and the hardness is from 550HV to 180 HV.And the influence of different pressure hardening parameters on the hardness zoning of gradient parts was also studied.By means of DIC technology and EBSD technology,the re-deformation ability of gradient specimens was evaluated and analyzed.The results show that there are obvious strain partition characteristics in the re-deformation process of gradient specimens.During the tensile process,the ferrite is responsible for energy absorption.The maximum strain of ferrite is0.32,and the orientation gradient of grain boundary is high.With the increase of deformation,dislocation density increases,a large number of small-angle grain boundaries are formed,the grain size decreases and the strain in micro-region is obvious.Martensite assumes the role of collision protection and there is no change in the grain orientation,which fits the application function of"hard zone anti-collision,soft zone energy absorption".