This is a bit old... but useful anyway.... found on Sumitomo web site.

April 16, 2001
Sumitomo Metal Industries, Ltd.

NAR - 410SM1 Stainless Steel for Bicycle Disc Brake Rotors Developed

Sumitomo Metals has developed NAR - 410SM1, a new kind of stainless steel for the disc brake rotors1) of downhill racing bicycles and mountain bikes manufactured by Osaka-based Shimano Inc. (Head office: Sakai City, Osaka Pref., President: Yozo Shimano), one of the world largest manufacturers of bicycle parts.
As the relevant technology is being considered for producing similar disc brakes for general bicycles, demand for NAR - 410SM1 is expected to increase remarkably.

1. Background
As downhill racing bicycles and mountain bikes require severe brake control, disc brakes are used instead of rim brakes using rubber pads on both sides of the aluminum rim. Under such severe conditions, it is necessary for stainless steel to be used to ensure the following:
(1) Appropriate hardness and anti-shock toughness for better brake control, and good corrosion resistance against muddy water, etc.
(2) Improved resistance to softening (resistance to toasting) 2 ) due to heating when braking.

2. Characteristics of NAR - 410SM1
For disc brake rotors, stainless steels such as SUS420J2 (0.3%13%Cr), SUS410 and SUS403 (0.1%C12%Cr) have been used, but they are not necessarily good enough to satisfy the above-mentioned requirements. Thus, SMI has developed NAR-410SM1, a stainless steel which features stable hardness and excellent corrosion resistance and hardly softens when heated during braking.
Based on SUS410 (0.07%C-12%Cr), NAR-410SM1 has improved corrosion resistance thanks to the addition of a small amount of special elements. On top of that, the addition of an appropriate amount of Mn (manganese), Ni (nickel) and other elements realizes high shock toughness that can be retained with increased hardness, contributing to improved resistance to toasting.

3. Future prospects
If disc brakes similar to those mentioned above are used for general bicycles by applying this technology, they can exhibit functional ability in terms of both corrosion resistance and brake performance to the fullest extent. Currently, Shimano Co., Ltd. produces about 120,000 rotors per month. Demand, however, is expected to increase several fold as this kind of rotor is increasingly used on more kinds of bicycles.
Furthermore, this newly developed stainless steel is expected to be used to make disc brake rotors for motorcycles as well.
In January 2001, Shimano Inc. received the Excellence award as part of the 8th Japan Stainless Steel Association prizes for its "bicycle disc brake rotors" employing NAR-410SM1.
SMI has applied for patents in several countries as well as in Japan together with Shimano Inc.

1) A disc brake controls rotation by pushing both sides of a disk fixed on the axis of rotation. The rotor is a part (disk) in the device.
2) When braking is continuously applied, the surface temperature of the rotor rises. If it reaches 500, the rotor material softens, resulting in the rotor surface becoming uneven. In addition, copper-based pads start melting and seize on the surface of the rotor, resulting in a worsening of braking performance and noise.