Dow Chemical is moving to buy Union Carbide in a huge $9.3 billion deal that would create a worldwide chemical industry giant, second in size only to DuPont. The deal still needs approval from the boards of both companies. Union Carbide is perhaps best known for the world’s worst recorded industrial accident, in which a pesticide plant in Bhopal, India, in 1984 leaked five tons of poisonous gas, killing more than 3,000 people and injuring 50,000.