Mild steel, remove it, Vee it out on both sides (remove the paint and degrease it with a good degreaser like say Purple Power, rinse thoroughly let dry and Vee it in both sides to the root and then MIG it with solid wire and Globular Transfer (which is high amperage MIG, over 200 amps with shielding gas (75-25 or straight argon), multiple pass until filled and grind it off smooth, repaint it and call it good). You will have to secure it in alignment on something square and solid, like a welding table.
Take about 15 minutes, start to finish.