A15 will do it, also known as ER70S-2, A18, ER70S-6 can also be used, as long as the materials clean it should weld no different to mild steel plate, DC Electrode Neg, and a thoriated or ceriated tungsten, clean the millscale of the "black" plate before you start, without knowing what you are doing its not really possible to give more advice, joint prep, pre and post heat, weld sequence etc.