Research On The Tailored Mechanical Properties Of Hot Forming High Strength Steel And Its Part

Recently, with the development of high strength steel hot forming technology, people begin focusing on different positions of one part satisfying individual properties to improve the comprehensive service performance during the collision. Because of this, a kind of hot forming high strength steel part with tailored mechanical properties has attracted more and more attentions in automobile industry. Based on hot forming technology, by controlling the technological parameters to adjust the cooling rate of quenching process, microstructures of the parts can be changed, so hot forming steel parts with tailored mechanical properties can be obtained. So this method has the key effect on improving service features of hot forming parts. In conclusion, the tailored mechanical properties of hot forming high strength steel and the service features of its parts are researched in this paper.Firstly, based on the designed flat tool fundamental test, the flat tool was segmented into heating and cooling zones, the relationships between cooling rates, strength and hardness of hot forming steel with tailored mechanical properties were discussed during the hot forming process. Based on the nonlinear regression equations, the cooling rate-hardness-strength exponential model were established respectively. The results show that under different conditions, the cooling rate realizes the gradient changing, so as the strength and hardness. With the effect of cooling rate, the micro structure evolution is from ferrite and pearlite, upper bainite, lower bainite, to martensite. At the same time the quantities of tempered martensite can be decreased by increasing the cooling rate, so the distribution of mechanical properties of the hot forming high strength steel and its part will be controlled. The established cooling rate-hardness-strength models can reflect the relationships between them truly and effectively.Secondly, based on the tailored mechanical properties of hot forming steel under different technological parameters, the different tailored mechanical properties three-part-distribution model was designed with a typical car body B-pillar, and to analyze and optimize with this collision model. The side collision performance of B-pillar with tailored mechanical properties was discussed by considering energy absorption and maximum intrusion during the side collision. The results show that to B-pillar parts, tailored mechanical properties of different positions have different effects on the side collision performance. When the middle strength is higher than end strength, B-pillar comprehensive side collision performance can be improved obviously by this typical tailored mechanical properties arrangement.Finally, based on the optimal tailored mechanical properties arrangement, the optimization mathematical model of typical B-pillar side collision was established by Latin hypercube design and response surface method. By multi-objective optimization using NSGA-II, the Pareto solution set with the best arrangement of tailored mechanical properties was obtained. Optimization results show that with the best arrangement of tailored mechanical properties obtained by multi-objective optimization, energy absorption of B-pillar increases and maximum intrusion of B-pillar decreases obviously. The ability of energy absorption to traditional hot forming steel parts is improved. Further prove that hot forming steel parts with tailored mechanical properties can improve the side collision comprehensive performance obviously, tailored mechanical properties can improve the collision performance of car body effectively.