There are four likely candidates for this project ...
HRS (hot rolled steel) usually specd as ASTM A36. Comes complete with a very hard, black, mill oxide (mill scale) covering. If you need to precision mark this material, the mill scale must be ground off (which is no fun) before Dykem can be applied. Cheap and available everywhere. Best for structural uses where it's bolted, welded, or riveted into buildings, bridges, etc. The last pick for a part that needs to be machined, unless you live somewhere that UPS does not deliver
CRS (cold rolled steel) ASTM 1018, is easier to machine than A36 as there is no scale to grind off. Problem is that 1018 comes with lots of internal stresses from the cold forming process, and these are released when one side of a part is machined. An OK material for lathe turning where equal amounts are removed from both sides of a shaft or pin, but a pain for flat work.
Alloy steels are much better to work with, both from a machinability standpoint & because of superior mechanical properties. The most popular alloy is 4140, readily available & not expensive. Since it contains chrome & molybdenum, it's often referred to as Chrome-Moly steel. Machines beautifully, especially if you specify DCF (decarb free) which has a nice, clean surface.
The best steel for a part like the clamp (IMO) is 4140 HT DCT (heat treated decarb free). Prehardened to about 30 HRc, meaning that all machining ops are easy to do - turning, boring, drilling, tapping, etc.
Some sources I've had good luck with (YMMV):
http://www.speedymetals.com/pc-1414-8316-58-x-4-4140-de-carb-free-heat-treated.aspx
http://www.alro.com/DIVSteel/STEELGUIDE_ALLYBAR_HR2.HTM
http://www.metalexpress.net/cgi-bin/index.pl?mod=catalog&ac=acDisProdList&material=Alloy%20Steel%20Hot%20Roll&form=Plate&cat=4140%20heat%20treat&min=0&max=0&sid=570b9216abc8787fbba661832be93e47
Metal Express is awfully good, especially if you phone (prices are lower by phone than by web). Alro is also top notch. Since the time & effort are about the same no matter which material you choose, you may as well get the one with the best properties. For the small piece you need, about $10 + shipping.
