Mild steel is not generally any specific grade of steel, and is also called 'commercial grade' steel. (aka crap) The C content is generally in a very wide range from about 0.05% to 0.2%, and is roughly equivalent to your 1010, 1015, 1018, 1020 etc. Most suppliers wont be able to accurately tell you the exact composition as its not highly controlled. To answer your question, its probably the one readily available steel that works roughly similarly to 'iron'. Bearing in mind that 'pure' iron does not realistically exist.
Lang