This is repeated from http://www.sukottoinc.com/steel_guide.php
I have done this because it answered the main question that I had, having seen swords for sale from a variety of sources recently.
There are many types of steel used in today’s production sword. The most popular is the 10XX series steel. However, 92XX series steel is quickly gaining popularity, creating a buzz due to its resilient but hard nature that is ideal for sword making.
10XX series steel
The 10XX nomenclature of these steels refers to the AISI/SAE designation for metal. The first 2 digits in this series refers to carbon steels with no major alloying agents. The second set of digits (as in a 1060) represents the carbon content of the steel in 1/100 of 1%. As an example, a 1060 designated steel would be a no major alloying carbon steel, with a carbon content of .6%.
It is common to make swords using the 10XX series of steel. Not only is it readily available, the steel itself is a close modern equivalent of the steels that were used in historical Japanese swordsmithing.
The higher the carbon content, the more rigid the crystalline structure that is formed after heat treatment will be. However, there is a tradeoff between hardness and ductility. When the cutting edge of the sword is too brittle, when the steel exceeds its ability to withstand external stress, it tends to shatter, as opposed to bending. This manifests itself in chipped edges, and in order to “fix” the blade, requires polishing in a way to take away material from the cutting edge, in order to make it smooth again. In contrast, a sword with an edge that is not hard enough would roll and dull itself on contact. Ideally, a Japanese sword should be both hard, and ductile.
In general, steels exceeding .9% carbon content (ie 1090 steel) is considered to be too brittle in sword making. Traditional swords ranged from .4% to .8% steel content.
1060 steels offer the best compromise between durability and hardness. When properly heat treated, 1060 steels, with its .6% carbon content, offers a very hard cutting edge, but is still soft enough to absorb the shock and stress forces during use.
Higher end swords, such as Hanwei use 1070 and 1080 steel in their sword production. It offers a highly rigid front end when it is heat treated, and is suitable for high performance cutting. A harder edge when sharpened has a greater ability to cut into material, and still retain its sharpness, while the softer back absorbs shock forces. Because of the slightly more brittle cutting edge, it is recommended for advanced cutters only. However, because of the stiffness, it maintains its cutting geometry even on a bad cut, and continues to cut through a target where as a softer steel would not be able to fully cut through.
9260 Series Steel
9260 spring steel is a new material in use for production swords. This steel also contains a .6% carbon content, but the 92XX series denotes the addition of silicone alloying agent. The benefits of a 2% silicone addition gives the sword a tremendous flex resistance, being able to return to center even after applying a bend of up to 90 degrees. The steel itself does not bend on its own, but rather after the application of a large amount of lateral force. In the dojo setting, this makes the 9260 steel an ideal material for cutting blades as a beater sword, for students to practice on. It is resistance to forces that normally would make a traditional sword bend, yet the cutting edge is still as hard as a full 1060 steel cutting sword.
Because of its properties, it makes it an ideal candidate for making tremendously light swords by utilizing bohi on the blade body, but yet, not reduce significantly its lateral stress resistance. Typically, where a 10XX series sword may increase its lateral load resistance by increasing the thickness of the blade body (i.e. increasing the niku), a 9260 blade body complete with bohi would still offer a significant advantage of fatigue resistance and return itself to the centerline without taking a set. This makes the 9260 steel a very good choice for sword production.
Another benefit to 9260 steel is that when differentially heat treated, it can also produce a very clean natural hamon. Unlike some specialty steels, the hamon is easily visible in proper lighting conditions. This hamon is all natural, and clearly visible, with no need for after production etching.