In this blog post, we’ll explore the characteristics and principles of steel, as well as why it’s used in so many different forms in our daily lives.
The Structure of Steel and the Principles of Strength
Have you ever heard of the “Damascus sword”? Often featured as an item in video games, this sword is a classic example of a high-quality blade used in the Islamic world during the Middle Ages. The “Damascus sword” was renowned for its exceptional durability—it was not easily broken—and was highly regarded on many battlefields of the time. But would you believe it if I told you that the material used to make this sword isn’t significantly different from that used in household cookware, such as pots and pans? In fact, both the “Damascus sword” and kitchen cookware are made from steel. Steel is also a core material in countless everyday products, such as building frames, cars, and refrigerators. So, what exactly is steel, which is used in such a wide range of fields, and what do its key properties—strength, ductility, and corrosion resistance—mean?
Steel is an alloy made by adding carbon to pure iron. An alloy is a material created by mixing two or more elements. To put it simply, it’s similar to a walnut bread with walnuts embedded in it. Iron atoms are arranged in a regular lattice structure, but this arrangement contains defects called “dislocations.” A dislocation is a linear defect where the atomic arrangement is slightly misaligned, and it plays a crucial role in the deformation of iron.
The reason steel deforms when subjected to an external force is because these dislocations move. For example, when a dislocation moves one unit along a regularly arranged atomic layer, the entire atomic arrangement shifts slightly, causing the material to deform. In other words, the bending or stretching of steel is the result of dislocation movement. Carbon enters the interior of the steel and hinders the movement of these dislocations. Just as walnuts inside bread hinder the movement of empty spaces, carbon atoms act as obstacles, preventing dislocations from moving easily. This is the same principle as when a very tall person stands in the middle of a line of soldiers—it makes it difficult for the entire line to move forward. Ultimately, carbon inhibits the movement of dislocations, preventing the iron from deforming easily.
In other words, carbon is added to iron to make it harder. Therefore, generally speaking, as the carbon content increases, the strength of steel increases. Here, “strength” refers to the property of a material not deforming easily when subjected to external forces. However, as strength increases, ductility tends to decrease. Ductility is the property that allows a material to stretch or bend without breaking. This is the same principle at work in a sturdy zelkova tree, which can break under a typhoon, whereas a reed bends easily without breaking. Reeds can be likened to a material with relatively low strength but exceptional ductility. Generally, steel has high strength, while copper and aluminum are examples of materials with superior ductility compared to steel. As such, strength and ductility are generally opposing properties.
Why the “Damascus Sword” Was Special
It is extremely difficult to significantly increase both strength and ductility simultaneously in steel materials. Nevertheless, medieval blacksmiths—in an era lacking modern theories of materials science—created swords that excelled in both strength and ductility based on their extensive experience and craftsmanship. They strove relentlessly to forge swords that were hard yet resistant to breaking, and the “Damascus Sword” is known as one of the results of those efforts.
Today, there are various theories regarding the manufacturing methods of the historical “Damascus sword,” but it is generally believed that a method involving the appropriate combination of materials with different properties—or the formation of a complex microstructure—was used to achieve both strength and toughness. This structure creates a unique and beautiful rippled pattern on the sword’s surface, which is also famous as a defining characteristic of the “Damascus sword.” Thanks to these characteristics, “Damascus swords” have long been renowned as exceptional blades that combine high strength with excellent toughness.
The Drawbacks of Steel and the Birth of Stainless Steel
Although steel is widely used in various fields due to its excellent strength and ductility, it does have a drawback: its tendency to rust, a phenomenon scientifically known as corrosion. Iron corrodes easily in environments where water and oxygen are present, gradually weakening over time. If the structural materials of buildings or bridges corrode severely, structural safety is compromised, which can lead to major accidents.
A prime example is the “Statue of Liberty” in New York City, which has undergone several rounds of restoration work over the years to address corrosion issues in its internal iron framework. This is because continuous corrosion of the internal iron structure can compromise the monument’s stability.
Stainless steel is a prime example of a material developed to overcome this drawback of iron. Stainless steel is an alloy created by adding a certain amount of chromium—in addition to iron and carbon—to provide excellent corrosion resistance. Depending on the application, various other elements such as nickel, molybdenum, and manganese may also be added.
Cookware used in the kitchen, such as pots and frying pans, is constantly in contact with water and is washed with water after use. Therefore, it is highly susceptible to corrosion. For this reason, most cookware is made of stainless steel to minimize rust formation.
Stainless steel naturally forms a very thin and dense oxide film when chromium combines with oxygen. This film protects the interior of the steel from the external environment and has the property of reforming in the presence of oxygen even if the surface is scratched. Thanks to this protective layer, the steel does not rust easily and maintains excellent corrosion resistance. Corrosion resistance refers to the property of withstanding corrosion well. This is why stainless steel is widely used in various industrial fields, such as automobiles, construction materials, medical devices, and kitchenware.
Although objects made of steel may look very different from one another, they all share the commonality that their fundamental material is steel. While the types and proportions of elements in steel are designed to vary in order to adjust properties such as strength, ductility, and corrosion resistance to suit specific applications, steel is widely used throughout modern society. It is no exaggeration to say that steel has always been at the center of human progress and technological innovation since the Industrial Revolution. Even in the countless products we use every day without giving them a second thought, steel stands as a core material that naturally sustains our lives, just like air or water.
