Nippon Steel Corp. said Thursday it will make an additional investment of over $1.3 billion in United States Steel Corp., in an apparent bid to help clinch its planned acquisition of the iconic American steelmaker. The move comes amid opposition to the deal from United Steelworkers, a major labor union. The latest investment, intended for two U.S. Steel plants, will add to the $1.4 billion Nippon Steel already promised to invest in the U.S. firm, the Japanese company said. Nippon Steel will invest at least 1 billion in Mon Valley Works in Pennsylvania to improve the plant’s competitiveness by …