Imports of corrosion-resistant steel from China are being unfairly traded at below market prices and will be slapped with a 256 percent tax, according to a preliminary finding from the Commerce Department that is expected to help bolster a domestic steel industry battered by cheap imports. The decision by the department is the first of several trade cases filed this summer by U.S. Steel Corp., AK Steel Holding Corp., Nucor and Steel Dynamics Inc. alleging their competitors in China and elsewhere are unfairly subsidized by their governments, allowing the foreign producers to undercut prices of U.S. firms.