This blog post explores why metal materials remain vital in the automotive industry and why they will inevitably play a central role in future technologies.
Materials we commonly encounter in daily life can be broadly categorized into metal materials, polymer materials like rubber or plastic, and ceramic materials like pottery or glass. Among these, I wish to discuss metal materials. Metal materials possess relatively high strength and excellent ductility, outstanding electrical and thermal conductivity, and the ability to reflect light while exhibiting specific colors. I will briefly explain why metal materials possess such superior mechanical properties.
Metal materials are the fourth most abundant element in the Earth’s crust. I will introduce steel materials, which are relatively familiar to us, and several materials that many materials engineers have been researching until recently. Steel materials are widely used in everything from construction materials to ships and home appliances. Among these, the most important field is likely the automotive industry, which is inextricably linked to daily life. In modern society, where energy conservation is a key issue, efforts continue to produce lighter, higher-strength vehicle bodies.
Currently, BMW has developed what is termed a “carbon car,” utilizing carbon fiber-reinforced plastic (CFRP) as its primary advanced material. However, the process of combining carbon fiber and plastic—two distinct materials—requires numerous steps. Mass production necessitates modifying existing equipment, resulting in lower price competitiveness compared to steel materials. Consequently, recent research has actively focused on returning to steel materials and improving them. Research is underway to create lightweight, high-strength steel materials by adjusting alloying elements and modifying heat treatment methods to improve the metal microstructure. This has led to the development of Dual phase steel and TWIP steel.
The mechanical properties of metallic materials can be broadly categorized into strength and ductility. Strength is the material’s resistance to deformation, while ductility is its ability to be drawn out and elongated. To be used as automotive materials, they must possess high strength to protect occupants during collisions and high ductility to facilitate manufacturing. While strength and ductility are generally inversely proportional, dual phase steel and TWIP steel are exceptional in exhibiting both high strength and high ductility. Understanding the deformation mechanisms of metallic materials is essential to explain this result.
Metals possess a crystalline structure where atoms repeat periodically like a chessboard grid. However, atoms are not perfectly uniformly arranged; sometimes a row of metal atoms is missing where it should be, or found wedged between other atomic rows. These defects are called dislocations. When external force is applied, the movement of these dislocations causes the metal material to deform.
To aid understanding, imagine laying a long carpet on the floor and trying to move it to the other side. Simply pulling it is difficult due to the friction between the long carpet and the floor. If you slightly lift one end of the carpet to create a fold, this fold does not touch the floor. As the fold travels to the opposite end, the carpet can move forward by the width of the fold. If we think of the rug moving as metal deformation, the crease acts like a dislocation. That is, if dislocations move poorly, strength increases; if they move well, ductility increases.
Dual phase steel is made by heat treating steel with a specific carbon concentration. It features a structure where hard martensite is mixed into a soft ferrite matrix, much like black beans scattered in white rice. Dislocations move easily in the ferrite, allowing deformation and high ductility, while the hard martensite inhibits dislocation movement, providing high strength. Imagine heavy stones placed on a thin carpet to grasp this concept.
TWIP steel is produced by additionally increasing the manganese content in the steel. This steel has an austenite structure with numerous twin boundaries. Dislocation movement is effectively blocked at these twins, resulting in high strength. Imagine a rug cut into several pieces and then stitched back together. If you try to move the folds as before, the movement is hindered more at the stitched sections than elsewhere. What happens at these stitched sections is similar to what occurs when dislocations pass through twins. TWIP steel, in particular, contains a large amount of relatively lightweight manganese, making it an optimized material for reducing vehicle body weight. Recently, POSCO in South Korea developed it using proprietary technology, drawing global attention. China is the largest producer of ferromanganese, the primary material in TWIP steel.
We have now covered the most prominent steel materials to date. Materials engineers often use the expression “designing steel!” This is because the microstructure changes depending on the amount of elements added to the steel or the degree to which the molten iron is cooled, leading to alterations in its mechanical properties. Through this process, steel is ‘designed’ to achieve the desired outcome. Even seemingly conventional steel materials can be reborn as products with immense added value when combined with materials engineering. As long as materials engineering research continues, the potential for existing materials to be re-evaluated remains infinitely open.
