Research On Hot Deformation Behavior And Hot Stamping Process Of Ultra High Strength Steel BR1500HS

Hot stamping process of ultra high strength steel has been regarded as one of the main feasible methods to achieve lightweight in “BIW” parts and improve its collision safety performance, which is a good solution to the problems of energy saving and good safety. In this thesis, the boron steel BR1500 HS produced by Baosteel is investigated then quenching experiments, Gleeble thermal simulation, numerical simulation of hot stamping process and hot stamping verification tests are carried out to obtain optimized parameters of hot stamping process. The main results are as followed.(1) In this research, effects of austenitizing process parameters on microstructure, the austenite grain size and mechanical properties of BR1500 HS are investigated. It is found that the optimum microstructure and mechanical properties can be achieved while heated at 920℃ and heat preservation 5min,which the microscopic structure is composed by uniform lath martensite and the tensile strength is up to 1789 MPa, elongation 7.5% and the product of strength and elongation is 1.34×104 MPa·%.(2)The non-isothermal tensile tests at high temperature are carried out at Gleeble3500 thermodynamic system, which provides reliable data and stress-strain curves at various initial deformation temperatures(900℃,850℃,800℃,750℃) and end deformation temperature(700 ℃) of BR1500 HS steel. What is more, microstructure analysis combined with the stress-strain curves is also fulfilled to study the rheological behavior in non- isothermal deformation process. The results indicate that the peak stress of non- isothermal deformation process is obviously higher than that of isothermal deformation reported in other literatures and with the increase of deformation temperature, the work hardening rate of BR1500 HS steel increases significantly. While the tensile samples deformation at high initial temperature(900℃), the deformation induced ferrite transformation(DIFT) is not obvious. But with the decreasing deformation temperature and strain rate, the DIFT significantly enhanced.(3)Based on the stress-stain curve obtained in the non-isothermal tensile experiment and some thermal parameters of BR1500 HS steel,PAMSTAMP 2G finite element analysis software is used to establish material model to do thermal coupling calculation of hot stamping process. Effects of the initial stamping temperature and holding time are investigated to obtain main parameters in hot stamping process by analyzing the temperature distribution and microstructure transformation of hot stamping process. The results indicate that when the initial stamping temperature is above 750℃ and holding time 15 s, the martensite phase fraction up to 90% and it exhibits good formability of the final part. While the initial stamping temperature 800℃ and holding time 10 s,the martensite phase fraction only 1.3% in the bottom of part, however, when the holding time extend to 15 s, the martensite phase fraction up to 98% of all position of the part.(4)Hot stamping experiment of V-shape part is carried out according to austenitizing process parameters, thermal simulation experiment and finite element simulation analysis results, which provides more reasonable parameters of hot stamping process of BR1500 HS steel with a thickness of 1.8mm. Under the conditions of this study, the austenitizing parameters is 920℃+5min, then quickly transfer to the mold within 10 s, make sure the initial stamping temperature above 800℃. Additionally, in the hot stamping process, the stamping speed is 100mm/s, the holding pressure is 50 tons and the quenching time should be 15 s with the flow rate 5mm/s of cooling water. The profile of hot stamping parts shows high dimensional accuracy with almost no spring back, except in the side of the part there is a significant thickness reduction(thinning 21.1%). Furthermore, the measurement data(thinning 25 %) and simulation results agree well. The results of microstructure observations and mechanical performance test of hot stamping parts indicate that the microscopic structure is mainly composed by lath martensite, which the tensile strength of typical test elements all higher than 1500 MPa. Thus, by comparison with the experimental results, the accuracy and reliability of the finite element model are verified.