DALLAS — The first nationwide strike at U.S. oil refineries since 1980 is spreading to two BP plants in the Midwest. The United Steelworkers union notified BP Plc. that workers at refineries in Ohio and Indiana will strike late Saturday night, joining a walkout that began this week at nine other refineries. A BP spokesman said that the company expects to continue operating with replacement workers and doesn’t expect a significant effect on production. About 3,800 steelworkers began a strike Feb.