Cobalt Special is a steel made by Takefu and consists of:
Some information from Michael Janich in a thread on Spyderco website:
The most notable element, as the name implies, is cobalt. For comparison, standard VG-10 contains 1.30-1.50% cobalt, N690Co contains 1.50%, and CPM-S110V contains 2.5%. At 2.00-3.00% cobalt, Cobalt Special will offer more of the qualities this alloy provides, specifically increases in strength and hardness and the ability to quench at higher temperatures. Cobalt also acts as a catalyst to intensify the benefits of other alloys.
As far as stain resistance, at 15.00-17.00% chromium, it is well above the threshold of a stainless steel and contains more chromium than VG-10's 14.50-15.50%.
Part of the challenge of providing information on this steel is that very little information is available on it in English. Takefu's own web site does not include it in their list of steels.
Some information from the Spyderco website:
Takefu is a premier Japanese steel company specializing in the production of steels used for household and industrial cutlery. They produce blades using clad steel. Clad steels are a composite of different metals some hard, some softer, fused or bonded to each other. Paring different steel types together elevates performance by combining the best characteristics of each material and where the steels overlay each other, a fine line develops. Traditional Japanese swords featured clad steels, layering tough harder steel with softer bendable ones which resulted in a steel combination that made them difficult to break or bend. Spyderco's MT09 Mule Team is Takefu's Cobalt Special steel enveloped within slices of SUS420J2. The two steels laminated as one boost toughness, improve corrosion resistance and generate lasting sharpness in a blade that is relatively easy to sharpen.