## 1 Introduction  
The most significant challenges that the automobile industry faces in the 21 st century are fuel efficiency improvement through weight reduction and driver/pedestrian safety. To satisfy these demands, a number of functional materials, including mild steel materials, vibration damping steel sheets, and clad metals, in addition to composite materials consisting of high-strength steel, aluminum (AI), magnesium (Mg), and other materials, are currently being employed in the industry. Research and development centering on lightweight technologies for material application are being conducted to solve problems associated with manufacturing costs and safety/durability concerns, which are obstacles to their use in mass production [1; 2; 3; 4]. As of 2019, the global market for lightweight metal materials was 35.7 billion USD and, since 2014, has been growing at a compound annual growth rate (CAGR) of 4.9%. Furthermore, to comply with U.S. Corporate Average Fuel Economy (CAFE) regulations by 2025, automakers in every country need to reduce the body weights of vehicles by 35% relative to those in 2006. Therefore, to address this requirement, lightweight materials and ultra-high tensile steels are continuously being developed. To reduce the weights of automobiles, industrial trends such as the diversification of molding materials targeting various difficult-to-form metal and non-metal materials and ultra-precision molding processes have been accelerating. A 10% reduction in vehicle weight improves fuel economy by 3-8% [5]. However, hybrid and electric vehicles, which have recently attracted attention from both the industry and the public as eco-friendly vehicles, tend to be 15-20% heavier than conventional gasoline vehicles because of their greater battery weights. Therefore, the use of lightweight technology is urgently needed in the battery-based eco-friendly automobile industry. Among the materials used in conventional automobiles, approximately 64% are steels, 9% are polymer and composite materials, and 8% are aluminums. Additionally, the power train, body, and suspension components account for 71% of the weight of a train body; therefore, reducing the weights of these parts is expected to be the most effective way of reducing the overall weight of the vehicle [6; 7]. On the one hand, as compared with that of mild steel, the weight of ultra-high-strength steel is 20% lower, Al is lower by 40%, and carbon fiber composites are lower by 50%. On the other hand, in terms of price, general steel sheets are the cheapest, followed by high-tensile steel sheets, Al, and carbon-fiber-reinforced plastics (CFRP). Figure 1 compares the relative weights and costs of lightweight materials against those of general steel plates [8]. Currently, in the industry, various materials are already being used to reduce the weights of transportation equipment, although Al and CFRP materials remain the most widely used. However, contrasting the urgent need to reduce vehicle weight in the auto industry, there is increasing pressure to enhance vehicle weight because of the strengthening of various collision safety laws and the ever-expanding inclusion of various convenience devices for consumer comfort. Therefore, there must be a compromise between these two necessities. Strategies for the weight reduction of vehicles can be broadly classified into three categories, namely the application of low-density materials, the rationalization of structures, and the development of new high-functionality materials. In the first category of strategies, metal materials are replaced with low-specific-gravity materials, such as non-ferrous metals or chemical materials. CFRP is a representative material for this type of strategy. However, this category of strategies introduces several difficulties because of the necessity to maintain mechanical properties equivalent to, or higher, than those of metal materials. The second category of strategies is structural rationalization, which involves the minimization of unnecessary reinforcement parts.