Wear resistance - : Just like it sounds, wear resistance is the ability to withstand abrasion. Generally speaking, the amount, type, and distribution of carbides within the steel is what determines wear resistance.
Strength: The ability to take a load without permanently deforming. For many types of jobs, strength is extremely important. Any time something hard is being cut, or there's lateral stress put on the edge, strength becomes a critical factor. In steels, strength is directly correlated with hardness -- the harder the steel, the stronger it is. Note that with the Rockwell test used to measure hardness in a steel, it is the hardness of the steel matrix being measured, not the carbides. This, it's possible for a softer, weaker steel (measuring low on the Rockwell scale) to have more wear resistance than a harder steel. S60V, even at 56 Rc, still has more and harder carbides than ATS-34 at 60 Rc, and thus the S60V is more wear resistant, while the ATS-34 would be stronger.
Toughness - : The ability to take an impact without damage, by which we mean, chipping, cracking, etc. Toughness is obviously important in jobs such as chopping, but it's also important any time the blade hits harder impurities in a material being cut (e.g., cardboard, which often has embedded impurities).
The knifemaker will be making a tradeoff of strength versus toughness. Generally speaking, within the hardness range that the steel performs well at, as hardness increases, strength also increases, but toughness decreases. This is not always strictly true, but as a rule of thumb is generally accurate. In addition, it is possible for different heat treat formulas to leave the steel at the same hardness, but with properties such as toughness, wear resistance, and stain resistance significantly differing.
Stain resistance(rust resistance): The ability to withstand rust (oxidation). Obviously, this property can be helpful in corrosive environments, such as salt water. In addition, some types of materials are acidic (e.g., some types of foods), and micro-oxidation can lead to edge loss at the very tip of the edge, over a small amount of time. In "stainless" cutlery steels, stain resistance is most affected by free chromium -- that is, chromium that is not tied up in carbides. So, the more chromium tied up in carbides, the less free chromium there is, which means more wear resistance but less stain resistance.
Edge holding: The ability of a blade to hold an edge. Many people make the mistake of thinking wear resistance and edge holding are the same thing. Most assuredly, it is not; or rather, it usually is not. Edge holding is job-specific. That is, edge holding is a function of wear resistance, strength, and toughness. But different jobs require different properties for edge holding. For example, cutting through cardboard (which often has hard embedded impurities), toughness becomes extremely important, because micro-chipping is often the reason for edge degradation. Whittling very hard wood, strength becomes very important for edge-holding, because the primary reason for edge degradation is edge rolling and impaction. Wear resistance becomes more important for edge holding when very abrasive materials, such as carpet, are being cut.
There are other properties that significantly effect how a steel performs:
Ability to take an edge: Some steels just seem to take a much sharper edge than other steels, even if sharpened the exact same way. Finer-grained steels just seem to get scary sharp much more easily than coarse-grained steels, and this can definitely effect performance. Adding a bit of vanadium is an easy way to get a fine-grained steels. In addition, an objective of the forging process is to end up with a finer-grained steel. So both steel choice, and the way that steel is handled, can effect cutting performance.
Manufacturing process: Cleaner, purer steels perform better than dirtier, impure steels. The cleaner steel will often be stronger and tougher, having less inclusions. High quality processes used to manufacture performance steel include the Argon/Oxygen/Decarburization (AOD) process, and for even purer steel, the Vacuum Induction Melting/Vacuum Arc Remelting (VIM/VAR) process, often referred to as double vacuum melting or vacuum re-melting.
Edge toothiness: Some steels seem to cut aggressively even when razor polished. For these steels, even when they're polished for push-cutting, their carbides form a kind of "micro serrations" and slice aggressively.
E. What's the "best steel".
Understanding these properties will get you started to fundamentally understanding steels and how choice of steel can effect performance. I often see people asking, what's the best steel? Well, the answer depends so much on what the steel is being used for, and how it's heat-treated, that the questioner can never possibly get an accurate answer. For a knife lover, it's worth spending a little time understanding steel properties -- only by doing so well he really understand what the "best steel" might be for his application.
Putting it all together, you can see how these properties might determine your steel choice. To pick on S60V and ATS-34 again, there seems to be a feeling that S60V is "better" in some absolute sense than ATS-34. But S60V is often left very soft, around 55-56 Rc, to make up for a lack of toughness. Even left that soft, an abundance of well-distributed vanadium carbides gives S60V superior wear resistance to ATS-34, at acceptable toughness levels. However, does that mean S60V is "better" than ATS-34? Well, many users will find edge rolling and impaction the primary causes of edge degradation for everyday use. For those users, even though S60V is more wear-resistant, S60V is also so soft and weak that they will actually see better edge retention with ATS-34! The S60V user can leave the edge more obtuse (raise the sharpening angle) to put more metal behind the edge to make it more robust, but now the S60V will suffer serious cutting performance disadvantages versus the thinner ATS-34 edge.
So, the next general rule:
Knowing the uses you'll put your knife to, and exactly how those uses cause edge degradation, will allow you to make a much better choice of steel, if you generally understand steel properties.
The properties of different steels will be laid out below. But in your search for the knife with the "best steel" for your uses, I always suggest you ask the makers of the knives you're considering which steels they would use. The knifemaker will usually know which steels he can make perform the best. And as pointed out above, heat treat is absolutely critical to bringing out the best in a steel. A maker who has really mastered one particular steel (e.g., Dozier and D-2) might be able to make that steel work well for many different uses. So never go just by charts and properties; make sure you also consider what the knifemaker can do with the steel.
III. ELEMENTS OF STEEL
At its most simple, steel is iron with carbon in it. Other alloys are added to make the steel perform differently. Here are the important steel alloys in alphabetical order, and some sample steels that contain those alloys:
Carbon: Present in all steels, it is the most important hardening element. Also increases the strength of the steel but, added in isolation, decreases toughness. We usually want knife-grade steel to have >.5% carbon, which makes it "high-carbon" steel.
Chromium: Added for wear resistance, hardenability, and (most importantly) for corrosion resistance. A steel with at least 13% chromium is typically deemed "stainless" steel, though another definition says the steel must have at least 11.5% *free* chromium (as opposed to being tied up in carbides) to be considered "stainless". Despite the name, all steel can rust if not maintained properly. Adding chromium in high amounts decreases toughness. Chromium is a carbide-former, which is why it increases wear resistance.
Manganese: An important element, manganese aids the grain structure, and contributes to hardenability. Also strength & wear resistance. Improves the steel (e.g., deoxidizes) during the steel's manufacturing (hot working and rolling). Present in most cutlery steel except for A-2, L-6, and CPM 420V.
Molybdenum: A carbide former, prevents brittleness & maintains the steel's strength at high temperatures. Present in many steels, and air-hardening steels (e.g., A-2, ATS-34) always have 1% or more molybdenum -- molybdenum is what gives those steels the ability to harden in air.
Nickel: Adds toughness. Present in L-6 and AUS-6 and AUS-8. Nickel is widely believed to play a role in corrosion resistance as well, but this is probably incorrect.
Phosphorus: Present in small amounts in most steels, phosphorus is a essentially a contaminant which reduces toughness.
Silicon: Contributes to strength. Like manganese, it makes the steel more sound while it's being manufactured.
Sulfur: Typically not desirable in cutlery steel, sulfur increases machinability but decreases toughness.
Tungsten: A carbide former, it increases wear resistance. When combined properly with chromium or molybdenum, tungsten will make the steel to be a high-speed steel. The high-speed steel M-2 has a high amount of tungsten. The strongest carbide former behind vanadium.
Vanadium: Contributes to wear resistance and hardenability, and as a carbide former (in fact, vanadium carbides are the hardest carbides) it contribute to wear resistance. It also refines the grain of the steel, which contributes to toughness and allows the blade to take a very sharp edge. A number of steels have vanadium, but M-2, Vascowear, and CPM T440V and 420V (in order of increasing amounts) have high amounts of vanadium. BG-42's biggest difference with ATS-34 is the addition of vanadium.