[Bill Harsey]

Some overview before we go into details later.
You kids see there are a lot of different types of tool steels used for knives.

Many of these tool steels were developed for some other hardened tool steel application than knives, some steels are further developed specifically for blade steels and not just for hand held knives but also stuff like wood chippers, wood milling, steel, plastic, paper and other material cutting blades and industrial scale food machinery as well as the entire range of tooling like drill bits and milling cutters and saws used to machine steel.

The reason we see so many different types of tools steels is that alloys are chosen for particular traits in specific applications or modified as some alloys become scarce or difficult to acquire because of world politics.

The first thing a knife blade has to do is be hard enough so the cutting edge doesn't roll over or break off when we try and cut something. Tool steels need enough carbon to form carbides when hardened.

Other alloys form different kinds of carbides and add to the strength of the matrix but the first and most important alloy of "high carbon" tool steel is carbon in carefully controlled amounts varying from .75% to well over 1% and in the class called "ultra high carbon" a steels carbon amounts can be from 1.25% to 2%.

(darn phone, have business call from Italy, be back later)

Edited to add, It's later:
The main areas of performance knifemakers and users are concerned with are:

Hardness
Toughness
abrasion resistance
stain resistance

These are all mechanical traits of a given tool steel that are both related to each other and quite seperate.

Anyone know the difference between hardness and toughness?