STEEL TECHNOLOGIES <STTX>, MITSUI SET VENTURE Steel Technologies Inc said it agreed to form a 50-50 joint venture with <Mitsui and Co Ltd> to make steel products. The venture, called Mi-Tech Steel Inc, will be set up to serve Japanese and domestic automobile and appliance parts makers in the U.S. A plant will be located near Murfreesboro, Tenn., and production is expected to begin in the fall 1987, the company said.