Rectangular Tubular Steel Sizes, Coremark Metals formerly Discount Steel. They are identified by size number, dimensions in mm, and weight in kg/m. 7×1. Includes nomenclature, tables, and professional guidelines. 00″ x 1. 0mm. Find customizable black iron tubes and mild steel pipes for various applications. Contact us today to find out how we can help you. Rectangular HSS Tube Steel Columns are available in a range of sizes with widths between 3”-8” (7. Great prices & free shipping on orders over $50 when you sign in or sign up for an account.