It depends on what proccess you are using (mig or tig) and straight Co2 will not work for tig welding. A mixture of 75/25 Co2/AR will work for mig welding.
Mild steel can be tig welded using straight argon with DC-SP.

Just to add to the other post,
Some (FCAW) fluxcore wires run straight CO2. We use ESAB 3/32" E70T-1 (flat/horz only) and 1/16" E71T-1 (all position) which are both dual shield 100%CO2 wires. We weld primarly all ASTM A36 or ASTM A572Gr50 (mild steel). Keep in mind, that fluxcore digs deep and will burn through on very thin materials (1/4" or less with the diameter wires we use). A lesser diameter wire will be needed on thinner materials. Some fluxcore wire are self-shielded and require no external gas coverage at all. Check the codes you are working to and see if they will allow self-shielded fluxcore. Some of our erectors use this type wire due to light wind and no need to lug a bottle up on the building.
John Wright

We use straight CO2 with solid wire. You get a little more spatter, you suck a little carbon out of the steel (lowers strength a little) but it's cheap and works well enough for us.
Bill