Carbon steel, alloy steel, tool steel: A little about 

the steel I use for my knives

There are many kinds of steel available.  Here is a little information about the different steels I typically use.  Steel is essentially an alloy of iron and carbon.  There are several additional alloying elements commomly used, and therefore hundreds of different kinds of steel.  For example, manganese is a sulfur scavenger, nickel adds toughness, chromium and molybdenum increase hardenability, silicon increases the strength, and tungsten increases wear resistance.  Please note that while I have listed the composition of the steels I often use below, there are slight variations in composition depending on who manufactures the steel.  I have also made knives with other kinds of steel even though I don't typically use them on a routine basis.

Also note that even though some of these steels have several alloying elements, they are all considered carbon steel.

1084 is a simple carbon steel containing 0.84% carbon,  0.75% manganese, 0.03% phosphorous, and 0.05% sulfur.

15N20 is an alloy steel with 0.75% carbon, 2% nickel, 0.4% manganese, 0.1% phosphorous, and 0.1% sulfur.  15N20 is often used for industrial bandsaw blades and is a good tough, flexible steel.

O1 tool steel is an alloy steel with  0.95% carbon, 0.5% chromium, 1.2% manganese, 0.3% silicon, and 0.5% tungsten. 

5160 alloy steel has 0.6% carbon, 0.8% chromium, 0.87% manganese,  0.03% phosphorous, 0.22% silicon, and 0.04% sulfur.  5160 is a very tough steel often used for car and truck coil and leaf springs.

80CrV2 is an alloy steel with 0.8% carbon, 0.5% chromium, 0.2% vanadium, 0.4% manganese, and 0.25% silicon.  80CrV2 is known for being very tough at a high hardness.

52100 is an alloy  steel with 1% carbon, 1.4 % chromium, 0.35% manganese, 0.25% silicon, 0.025% phosphorus and 0.025% sulfur.   52100 is a very hard and tough steel commonly used for ball bearings and roller bearings.

The final properties of a particular steel is a trade-off between hardness and toughness, and is highly dependent on the heat treat process. Also note that different steels, as well as different knives for different uses, may have a different optimal hardness.  I do all of my own heat treating.  I test most of my blades for hardness (Rockwell C scale, HRC) to ensure the heat treating was done correctly and the blade has the right properties for its intended use.  The hardness is reported in the knife description, e.g. 59 HRC.  In some cases the blades are not tested.  This could be for different reasons, such as when a san mai construction or a differential hardening technique was used.  In these situations the hardness is provided as an estimated range based on previous data for the hardness of a given steel obtained using the appropriate heat treating conditions.  For these knives, the hardness is given as an estimated range of values, e.g. estimated hardness 58-60 HRC.

Note that carbon steels are not stainless and will tarnish or rust if not cared for properly.  Please see my "Knife Care" section for some information about the care of a carbon steel knife.