Improvement of Weldability of 1 GPa Grade Twin-Induced Plasticity Steel

Abstract

Applications of high-strength steel to car bodies with higher than 1 GPa grade tensile strength have dramatically increased. Twin-induced plasticity (TWIP) steel maintains high strength and secure high ductility, featuring a high content of Mn. In spite of these advantages, the resistance spot weldability is poor. To improve the weldability, conventional AC and MFDC resistance spot welding, and other various types of joint technology have been required and developed. In this work, MFDC constant power control (CPC) welding was applied to improve the weldability of 1 GPa grade TVVIP steel, and the welding characteristics were analyzed. To investigate the fundamental weldability of TVVIP steel, tensile shear strength, suitable welding range, and welding signal were analyzed. Compared to MFDC constant current control (CCC) welding, CPC welding has the advantage of reducing expulsion at the early stage of the welding process; therefore, it is possible to apply higher heat input to the weld. As a result, it was found that larger nugget size and higher tensile shear strength were obtained in CPC welding than CCC welding. To compare the characteristics of these two welding methods, welding signals were analyzed and high-speed images were used for observing the difference in the nugget growth mechanism. Furthermore, this work evaluated the adaptability of CPC welding according to elecforce changes and coating conditions.