Formability Improvement Technique for Heated Sheet Metal Forming by Partial Cooling

Abstract

A forming process for heated sheet metal, such as hot-stamping, has limited use in deformable shapes. Higher temperature areas which have not yet come into contact with dies are more easily deformed; therefore, local deformation occurs at these areas which leads to breakage. To improve the formability of heated sheet metal, a deformation control technique utilizing the temperature dependence of flow stress is proposed. This technique can suppress local deformation by partial cooling around potential cracking areas to harden them before forming. In order to apply this technique to a variety of product shapes, a procedure to determine a suitable initial temperature distribution for deep drawing and biaxial stretching was developed with a coupled thermal structural simulation. In this procedure, finite elements exceeding forming limit strain are highlighted, and an initial temperature distribution is defined with areas of decreased temperature around the elements to increase their resistance to deformation. Subsequently, the partial cooling technique was applied to a deep drawing test with a heated steel sheet. The results of the experiment showed that the proposed technique improved 71% in the forming limit depth compared with results obtained using a uniform initial temperature distribution.

New Developments and Future Trends in Low-Temperature Hot Stamping Technologies: A Review

“…Beside this, the die temperature gradually increases during the production of parts quenched from high temperature, leading to severe wear that reduces the lifetime of the dies [70,71]. To overcome this limitation, as shown in Figure 6, Balint et al [72] and Ota et al [73] have proposed a novel process whereby the austenitised steel blanks are pre-cooled to a temperature just above the martensite start temperature before stamping. At this lower forming temperature, strain hardening can be enhanced, improving the drawability and formability; the productivity can also be increased by shortening the cooling cycle time.…”

Development of Hot Stamping for Automotive Panel Parts with Step-Shaped Wall

“…It is reported that cylindrical hole expansion by hot stamping prevent the cooling of the hole edge and increases the forming height. 11) As a method for improving the deep drawability of hot-stamped parts, Ota et al 12) proposed a method whereby air is injected onto the vulnerable upper wall of the step-shaped wall in the part being formed in order to increase the fracture strength of the part in contact with the punch shoulder. Also, reports have been published about technologies for preventing the temperature drop of the flange of the part by providing a gap between the die and the holder.…”

Minimisation of Heating Time for Full Hardening in Hot Stamping Using Direct Resistance Heating

“…Nürnberger et al [20] studied the hot stamping of heat treatable steel 22MnB5 using water-air spray cooling. Ota et al [21] improved the formability by partial air cooling of potential cracking regions of blank to harden them before forming. Zhao et al [22] showed high hardness and minimum springback of a hot-stamped part by using rapid cooling to a forming temperature between 700 and 750 • C after ejecting from the furnace.…”