Dr. K.
Knife Expert
What Exactly is Steel?
For a material that we see, use, and ride in every day, it's surprising how little most people know about steel, even though it is the underpinning for many of our modern conveniences. In this short piece I would like to give a broad picture of what steel is composed of, and how it's made. Then in future discussions, I intend to get specific and look at various types, their composition, how they are hardened and produced, and several other items of interest.
To start with, steel in its most basic form, is over 98% Iron. Iron ore is mined, then reduced back to its pure metal form in a "melt", via the use of something called a blast furnace. There are several types of blast furnaces, but they all basically use iron ore, something called coke(derived from coal), and limestone. These ingredients are literally heated and melted together. Two products are produced; molten iron (called pig iron) and slag(waste). This is a continuous process that goes on round the clock until the furnace surface wears to the point that it needs to be shut down and rebuilt. Ports at various places in the furnace stack draw off the molten iron and slag as new material is added.
This pig iron is then used in further processing as the main ingredient for basic steel, in something called an open hearth furnace, where carbon is introduced to form the molten steel.
The first steel was very simple, and consisted of iron and a small amount of carbon. This "impurity", when added to the mix, did wonderful things. It toughened the iron, made it stronger, and allow it to hold its shape better. However, this is only part of the story. To get the carbon to combine properly with the iron in the first place was not a simple matter. Iron(Fe) and carbon(C) can and do combine to form FeC, but this is not what makes steel what it is. For now, let me say that steel has more to do with atomic lattice structures and their adjustment than to chemical combining.
So, how much carbon ends up in steel? If you look at a table of various steels, especially the steels used for knives, you will quickly note that 1.5% seems to be the maximum for any normal steel in the table. There is a reason for this, the most important being that it's all the carbon this molten pig iron will hold and combine properly with when it is later cast and cooled, or rolled as new steel. More than that is basically rejected. This restriction has been circumvented somewhat by the new CPM steels which will be the topic of another discussion. Carbon will not only combine with iron, but it will also form carbides in steels that have been further modified with other elements as we shall see in later articles, so it’s advantageous to get more carbon into steel for specialty applications such as steel for knife blades and other severe duty applications.
Steels with a carbon content of 0.5% or more are considered “high carbon” steels and are superior to "mild"(low to medium carbon) steel for knife blades. The more carbon you have, the more steel can be hardened and yet not get too brittle.
As you can imagine, something as important to our culture as simple mild steel has volumes written on its manufacture, properties, and uses. That doesn’t even count the huge amount of information on the alloy steels.
Dr. K
