Abstract
This study involves performing direct and indirect hot press forming on ultra-high-strength steel (UHSS) boron steel sheets to determine formability. The indirect hot press process is performed as a cold deep drawing process, while the direct hot press process is performed as a hot deep drawing process. The initial blank temperature and the blank holding force are set as parameters to evaluate the performance of the direct and indirect deep drawing processes. The values of punch load and forming depth curve were obtained in the experiment. In addition, the hardness and microstructure of the boron steel sheets are examined to evaluate the mechanical properties of the material. The forming depth, maximum punch load, thickness, and thinning rate according to blank holding force were examined. The result shows that a larger blank holding force has a more significant effect on the variation of the thickness and thinning rate of the samples during the drawing process. Furthermore, the thinning rate of the deep drawing part in with and without fracture boundary was respectively examined.
Highlights
Boron steel currently represents the ultra-high-strength steel (UHSS) applied in the automotive industry because of the demand for higher passive safety and weight reduction
In direct hot deep drawing, the drawing process carried out with different initial blank temperatures ranged from 850–950 ◦ C, while in indirect hot deep drawing, the blanks were firstly pre-formed in room temperature, and the cold pre-formed parts were heated to 900 ◦ C and quenched in water
When the steel sheet was formed in this equipment, the linear variable differential transformer (LVDT) measured the punch load according to punch stroke
Summary
Boron steel currently represents the ultra-high-strength steel (UHSS) applied in the automotive industry because of the demand for higher passive safety and weight reduction. Naderi et al presented hot stamping as a non-isothermal, high-temperature forming process, in which complex ultra-high-strength parts are produced, with the goal of no springback [5]. The results showed that the material has excellent formability and can be formed into a complex shape in a single stroke He studied the tensile strength and microstructure change during hot stamping. Et al provided information on cold and warm blanks of the quenchable boron steel 22MnB5 [8] From their experimental research, it can be concluded that higher quality and more economical production can be achieved by adjusting the blanking process parameters for the commonly used ultra-high-strength steel sheet 22MnB5. This study, using a boron steel as the blank material, investigated the formability of direct and indirect hot deep drawing under different blank holding forces. The thinning rate, microstructure, and hardness at different positions of the drawn part were examined for direct and indirect hot deep drawings
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