Metallic materials still play an important role in the automotive industry thanks to their excellent mechanical properties, which combine rigidity and ductility, and advances in materials science are making steel lighter and stronger. Dual phase steel and TWIP steel are the result of this research, and the possibilities for steel materials in the future are endless.
The materials that we can easily encounter in our daily lives can be categorized into metal materials, polymer materials such as rubber and plastic, and ceramic materials such as ceramics and glass. Of these, I’m going to talk about metallic materials. Metallic materials are relatively hard, have good stretching mechanical properties, are excellent conductors of electricity and heat, and have the ability to reflect light with a specific color. I’d like to briefly explain why metallic materials have such good mechanical properties.
Metallic materials are the fourth most abundant of the elements that make up the Earth’s crust. I’d like to introduce you to some materials that are relatively familiar to us, such as steel, and some that have been studied by many materials engineering students until recently. Steel is used in everything from building materials to ships to home appliances, and perhaps most importantly in the automotive industry, which is inextricably linked to our daily lives. In the modern world, where energy conservation is a topical issue, there is an ongoing effort to create lighter and stronger car bodies.
Currently, BMW has developed a so-called “carbon car” based on a high-tech material called carbon fiber reinforced plastic. However, the process of compounding carbon fiber and plastic requires many processes and requires changes to existing facilities to produce in large quantities, making it less competitive in price than steel. For this reason, researchers have recently returned to steel materials to improve them. Research is underway to improve the metallographic organization by adjusting the alloying additives and changing the heat treatment method, resulting in lighter and stronger steel materials. As a result, dual phase steel and TWIP steel have been developed.
The mechanical properties of metallic materials can be broadly categorized into strength and ductility. Strength is the ability of a material to resist deformation, while ductility is the ability of a material to be stretched over a long period of time. To be used in automobiles, a material must have high strength to protect the driver in the event of a collision, and high ductility for ease of manufacturing. In general, strength and ductility are inversely related, but dual-phase steel and TWIP steel are the exception to this rule. To explain this result, it is necessary to understand the deformation mechanism of metallic materials.
Metals have a crystal structure in which the same atoms repeat periodically, like the lattice pattern of a checkerboard. However, not all atoms are arranged perfectly evenly, and it is not uncommon to find rows of metal atoms that are not where they should be or are sandwiched between other rows of atoms. These defects are called dislocations. Because of the movement of dislocations, metallic materials deform when subjected to an external force.
To illustrate, consider a long rug lying on the floor and trying to move it to the other side. Simply dragging it will not work because of the friction between the rug and the floor. If you lift the end of the rug slightly to create a crease, the crease doesn’t touch the floor, so when it reaches the other end, the rug can move forward by the width of the crease. If you think of the movement of the carpet as a deformation of the metal, the crease acts as a dislocation: if the dislocation moves poorly, it increases strength, and if it moves well, it increases ductility.
Dual phase steel is made by heat treating steel with a specific carbon concentration. It consists of a matrix of ferrite, a soft steel, mixed with martensite, a hard steel, like black beans in white rice. The easy movement of dislocations in the ferrite allows it to deform easily, resulting in high ductility, while the hard martensite prevents the movement of dislocations, resulting in high strength. Think of it like heavy stones in the middle of a thin carpet.
TWIP steel is made by increasing the amount of manganese in the steel. This steel has a structure called austenite and has many interfaces called grain boundaries, where the movement of dislocations is effectively blocked, giving it high strength. Consider again a carpet that is divided into pieces and stitched together. If we move the folds as we did before, the stitched parts will be more resistant to movement than the other parts. What happens at the stitches is similar to what happens when a dislocation passes through a dipole. In particular, TWIP steel is a material optimized to reduce the weight of the car body by containing a lot of manganese, which is relatively low in weight, and has recently attracted global attention for its unique technology developed by POSCO in South Korea. The largest producer of manganese iron, the main ingredient in TWIP steel, is China.
So far, we have explained the steel materials that are attracting the most attention. Materials engineering students often use the expression “design the steel!”. This is because the amount of elements added to the steel, or the degree to which the steel is melted and cooled, changes its microstructure, which in turn changes its mechanical properties. This process is called “designing” the steel to achieve the desired result. Steel, a seemingly boring material, can be transformed into something with enormous added value through materials science. As long as materials science research continues, there is always the possibility of rediscovering existing materials.
