Circular saw blades (this are of the 10" size range?) that have carbide tips are usually NOT good steel for blades. If you have circular saw blades without carbide tips they might well be good steel, possible L6? If you really wish to use non-carbide tipped blades, you might try heating a coupon (perhaps 1" x 3" test piece?) just past non-magnetic, quench in canola oil and check to see if a file skates. "IF" the file skates indicating it's hard, then temper at 400ºF, then check again to see how well the file cuts the test coupon.
You might consider breaking a 1" section off prior to tempering to check grain. Clamp in vise and hit with hammer. The grain should have a "gray" look to match a broken file indication of small grain.
Best thing is to buy some 1080 type steel. It won't cost much, and you'll know exactly what you got. BUT - I got my start using "mystery" steel and some of those turned out pretty good knives, but finally moved known steels, mostly stainless because my wife didn't like the patina of carbon steels.
Good luck.