CPM-M4 consists of:
From Spyderco's website:
CPM-M4 is a high-speed, wear resistant, tool steel. It is non-stainless steel. Its high content of Molybdenum and Tungsten gives it a fine grain size, greater strength, hardness and toughness. It is a triple temper heat treat to make it as tough as possible without brittleness. The grinding combined with the triple temper heat treating process can cause warping up to 50 thousandths (.050) of an inch which is not enough to affect the performance of the knife. CPM-M4 will provide high edge retention and impact resistance with an Rc of 58-63; you'll also find it's relatively easy to sharpen.