AISI 321 Stainless Steel Round Bars Specification. SS 321 Flat Bars: 1 / 2" to 10" in thickness range of 1.6mm to 150mm, Custom Size Stainless Steel Flats available. Finish: Black, Bright Polished, Rough Turned, NO.4 Finish, Matt Finish, BA Finish. Length: 1 to 6 Meters, Custom Cut Lengths. Form : Round, Square, Hex (A/F), Rectangle, Billet, Ingot, Forging Etc.