S355J0 is a common unalloyed steel with a yield of min. 355MPa and charpy's at 0°C (J0). It is comparable to A572 grade 50. Machining shouldn't pose a problem.
Find a low carbon steel (< 0.20% C) on one of the machinability rating tables from Sandvik, etc. This is not a free-machining grade, but it is easily machined.