M390 is a steel made by Böhler Uddeholm and consists of:
Some information from the Spyderco website:
The Austrian's Böhler Group merged with Sweden's Uddeholm Steel Works in 1991, creating the world's largest tool steel manufacturer, Böhler Uddeholm. Specializing in aviation/aerospace and automotive steels, they also produce tool steels used in the manufacture of consumer goods including electronics and cutlery. Through a powdered metallurgy processes they produce M390 Microclean steel which is highly sought and well suited to today's high-performance cutlery. M390 is a martensetic chromium steel highly resistant to corrosion. The powdered granules of Chromium and Vanadium in the steel are small, consistently sized, and finely dispersed in the steel's matrix (mix) making it exceptionally homogenous. This means for the consumer cutting with M390 will experience a high-level wear and corrosion resistance and the blade steel can be polished to a mirror-like finish.