Design And Manufacture Parts With Ultra High Strength Steel Based On Tailored Properties Distribution

Currently, the automobile industry is facing with the requirements of energysaving, safety and environmental protection. At the same time, also these requirementsare the future development direction of the vechiles. Using the ultra high strengh steelto manufacture automobile parts is one of the effective ways to slove these problems.However, the steels offer very low levels of ductility which limits their application asenergy absorbing structures. At presents, the key collision parts in BIW are mostlycomposed of tailor welded blank. These parts are obtained by hot stamping tech usingdifferent blank with different thickness, coating or different materials. However, due tothe processing limitations, the tailor welded blank has some disadvantages, such asstress concentration at the welds where the thickness change suddenly, and the singlematerial properties that can not meet the requirements of a variety of collision strengthdistribution and so on. Therefore, in order to improve the crashworthiness, this paperhas studied the tailored tempering process and the hot stamping workpiece withcontinous changing gradient properities are obtained. Firstly, the relationships betweenthe hot stamping process parameters and the properties and the microstructs ofworkpieces were studied. And then workpiece with tailored properties weremanufactured by changing the heating temperatures. Secondly, four kinds of heattreatment methods were designed to realized the workpieces with tailored properities.By comparing the four methods, the results show that blank tempering method was themost effective method under our experimental platform. And the forming accuracy ofthe workpiece was well. Also it has a significant gradient.Based on the experiment results above, the relationship between crashperformance and the single gradient properities, tailored properties are studied througha numerical simulation method. Crash performance test was conducted by Charpyimpact tester. The experiment was same as the dynamic three-point bending process.The results show that, with the increase of tensile strength of the hot formed parts, the energy absorption of the workpiece was increased, while the springback angle wasincreased too. Then a FEM model was established referring to the modified Charpyimpact tester. Simulation and experimental results were basically simlar in rules, butthere are some differences. By comparing the simulation and experimental results, wecan draw the following conlusions: With the increase of tensile strength of hot stampedparts with single property, the energy absorption increased. But the springback anglealso increased which is not good for cushioning and energy absorption for cars. But thehot stamped parts with tailored properities were superior to the single propertyworkpiece in terms of the springback angle, and are not good as the parts with samelevel of energy absorption. This is because the energy absorption zone increased withthe increase of austenite region.