| With the development of the automobile industry and people’s attention on energy saving and emission reduction,the automobile lightweighting has drawn more and more attention.The automobile steel has been developed in the direction of ultra-high strength.Ultra-high-strength steel is difficult to use the traditional cold forming precision forming,cold stamping parts easy to crack,but also rebound,resulting in parts of the shape stability and dimensional stability of the deterioration of the body subsequent assemblly process.In order to solve this problem,hot stamping high strength steel came into being.The purpose of this paper is to take a V-shaped thermoformed steel produced by a domestic steel mill as the research object,and to establish the deformation resistance model of the thermoformed steel as an important basis for formulating the reasonable rolling schedule for the hot stamping forming steel;This paper also studies the stamping process characteristics of the hot stamping steel in order to guide the hot stamping production,On this basis,a reasonable stamping process was devised to produce the car door beam with impact beam.The variation of microstructure and properties under different stamping process was studied.The details are as follows:(1)The peak activation energy and steady-state activation energy of hot stamping steel are 312.377kJ/mol and 297.866kJ/mol,respectively.The deformation resistance model of the hot stamping experimental steel can be expressed by the piecewise function of(3.13),where σp,σs,εp can be calculated by(3.14)～(3.16)and C=0.4551,C1=4.1610.The predicted value of this model is in good agreement with the experimental value.(2)In this paper,the static continuous cooling transformation of hot stamping experimental steel was studied.Ac1=728℃ and Ac3=817℃ for hot stamping steel.The critical cooling rate of all-martensite transformation was 30℃/s.The shorter the sheet transfer time from the furnace to the die,the better the stamping performance and the greater the cooling rate during the stamping process to avoid non-full martensitic microstructure after the stamping.(3)The door beam was successfully punched for trial production.The influence of hot stamping process parameters on the microstructure and properties of stamping parts was studied.When the heating temperature is 910℃ and 930℃,the microstructure of stamping parts are all martensite,but the grain size is finer and the performance is better when the heating temperature is 910 ℃.Under the premise of ensuring austenitizing,the holding time should not be too long,to avoid coarse grains,increase the production cycle,thereby reducing production efficiency.(4)Compared with the conventional B hot stamping steels,the original austenite grains of the V-shaped hot steels studied in this paper have been obviously refined.The martensite of the door impact beam stamping has a lath structure,which is typical topographical characteristics of the stamping martensite,and the martensite substructure contains a large number of dislocations.In addition,car door impact beam stamping carbide precipitation in the microstructure,and the carbide is very small short rods,dispersed in the martensite lath. |
