Now these elements are added in order to make various steel/alloys for different purposes, some of which can be used to make excellent knives. These purposes can include sustained hardness at elevated temperatures (known as "red hard"), increased wear resistance, extreme shock/impact loading and other factors. Knowing what is added for which application can come in handy when you are trying to figure out what steels to use, or in identifying a "mystery metal".
Aluminum: (Al): Aluminum is used to primarily control grain size and as a de-oxidant. Al killed steels show a high order of fracture toughness. Al is used primarily in Nitriding steels where the presence of Oxygen will be detrimental to the process.
Boron: (B): Boron's unique ability is to increase the hardenability of steels in very small amounts (0.0005%). This effect on hardenability is most pronounced in the simpler, lower C content alloys but it diminishes as the C content is raised, until this effect becomes negligible. Boron is only used in Aluminum killed steels. Unlike many other alloying elements B does NOT increase the ferrite strength of the steel, hence, it will promote machining and forming operations at various hardness levels.
Carbon: (C): This is the most common element added to steel, and perhaps the most important, although not many folks consider it a alloying material. Raising the Carbon content will raise the hardness and strength of the material, but too much C can cause embrittlement.
Chromium: (Cr): This element has been given more or less a "bad reputation" as far as blade forgers go, and while its presence can be a detriment in terms of blade "toughness" in the higher amounts of say a "stainless" steel alloy, in small amounts (less than 5% or so..) it can enhance the ultimate strength of steels, along with lowering the transformation temperatures, increasing deep hardening, abrasion resistence, reduces scaling at elevated tempertures while working and a few other "nice things". It can, however cause problems while working in the "stainless" materials as it can cause grain problems when improperly worked. All in all..a small bit og this can go a low way in improving performance.
Cobalt: (Co): Cobalt will increase the density of steels as it is dissolved within the iron base. It also raises the quenching temperature. Amounts up to 14% help to stabilize some other elements within the alloy, such as Cr, W, as it helps to retard the breakdown of Cr and W carbides.
Copper (Cu): Copper is added to steels primarily to "improve" corrosion resistence.. Amounts vary between 0.20% to 0.50%. It can cause surface oxidation that can cause some problems when working.
Lead (Pb): Lead when added in 15 to 35 pts content, it enhances the steel's machining characteristics. It does NOT have any significant effect on the hardenability of said steels