The manufacture of “green steel” moved one step closer to reality Friday as Massachusetts-based Boston Metal announced a $120 million investment from the world’s second-largest steelmaker, ArcelorMittal. A non-profit called ResponsibleSteel, for example, brings together stakeholders from up and down the supply chain — mining to finished steel products — to cooperate on cleaning up steel. Nearly 90% of an offshore turbine’s weight is steel, and each one, including the foundation, requires roughly 180 tons of steel per megawatt, according to the industry group American Clean Power