What type of steel are circular saw blades made of

All kinds. There are thousands of grades of steel and any given circular saw blade manufacturer will use one or another alloy for their blades. Modern high quality circular saw blades have tungsten carbide teeth brazed on to some sort of carbon steel alloy blade. The steels they use aren't the highest grade tool steels because they don't have to be, all the work is being done by the carbide teeth. Paradoxically the cheaper all steel blades use higher grade high carbon steels. If you're asking because you want to make a knife or something, avoid any kind of blade with carbide teeth. A $10 HCS 10-in circular saw blade is actually made of a much higher grade steel.
If the steel itself is the cutting material, then it is probably HSS (M2 high speed steel) or just carbon steel, depending on the materials it was designed to cut. For saws where the steel is not the cutting material, e.g. tungsten carbide, it may also be HSS, but would probably be something cheaper. There are also blades where the blade body is carbon steel and the brazed bits are M2.