Abstract
The automotive industry has tried to employ ultra-high-strength steel (UHSS), which has a higher strength with a thinner thickness. However, because of its low formability, there is a limit to the use of UHSS in industrial applications. Even though the hot-press-forming method can resolve the formability problem, elevated-temperature conditions lead to side effects—heat transfer and productivity issues. This work presents the concept of an infrared local-heat-assisted cold stamping process. Before the forming process, local areas, where the formability problem occurs, are locally heated by the gathering of infrared rays and cooled to room temperature before delivery. Since the heat treatment is completed by the material supplier, the stamping companies can conduct cold stamping without new investments or the productivity issue. In this work, a heat-assisted cold V-bending test was conducted with a martensitic (MS) 1.5 GPa steel, the CR1470M steel provided by POSCO. The heating effects on the microstructure, hardness, and local ductility were also observed. Finally, a commercial door impact beam was successfully manufactured with the present method. In this application, only a targeted small area was heated. The results show that the present method can improve the formability and springback problems of MS steel in the stamping process.
Highlights
In order to meet the requirement of reducing the weight of the car body, steel companies have developed ultra-high-strength steels (UHSSs) that have a higher strength with a thinner thickness
The formability problem has been an obstacle to the application of the UHSSs
These results show that heat-assisted cold forming in 400 and 600 °C conditions can resolve both the formability and springback problems, and the results from the 400 and 600 °C
Summary
The formability problem has been an obstacle to the application of the UHSSs. In order to improve the formability, some studies have introduced heating methods [1,2,3]. One of the widely used methods is hot press forming [4,5,6,7,8]. Hot press forming can manufacture auto parts having about a. 1.5 GPa strength without factures or springback. This method requires quenching time to make the martensite microstructure, and austenitization is needed, which requires maintaining a high-temperature condition during the heating process, resulting in high energy consumption [9]
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