Effect of Deformation on Microcrack Depth in Hot Press Formed Part with Galvanized Steel Sheet

• Published in: ISIJ International Vol. 59; no. 7; pp. 1295 - 1303
• Main Authors: Choi, Jong Won, Oh, Kyung Seok, Kim, Hong Gee, Kim, Dong Jin
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 15.07.2019
• Subjects: galvanized steel; hot press forming; microcrack; Side-Forming
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2018-734

Surface cracks or microcracks have been reported as one source of fatigue life deterioration. Propagation of microcracks into the steel substrate may appear on a deformed part when zinc-coated steels are used for direct hot press forming. This work investigated crack occurrence in galvanized steel sheets with coupled thermo-mechanical finite element simulations and realistic HPF experiments. This study used a two-dimensional hat-shaped tool called Conventional–Forming to effectively feature a cross-section of hot press formed parts in a vehicle. Microcracks occurred on the product wall-outside when the Conventional-Forming die moved vertically and the tool contacted the material. The depth of microcracks in the deformed zone where compressive-bending and unbending followed by a straightening occur is highly dependent on the magnitude of effective strain. Therefore, it is presumed that smaller magnitudes of effective strain from forming can reduce the microcrack depth on the workpiece. To identify ways to lower the effective strain, the study focused on the tool contact and bending curvature of the workpiece during forming. Consequently, Side-Forming, in which a material undergoes deformation behavior from a die entering from an oblique direction, was proposed. With other conditions unchanged, the numerical calculations and experimental results indicated a lowered effective strain and a subsequent significant reduction in microcrack depth. Moreover, the required qualities for hot formed products such as hardness and shape accuracy could be also achieved.