black welded square steel tube Material:Q195 Q235 St52 10# 20# Standard:BS JIS GB ... DIN Cert:CE,BV,SGS,API Bright,Oiled, black welded square steel tube Specification:1. Outer diameter:15*15-400*400mm2.Wall ... ,SGS,others is appliable.Processing technic:1.Process method:cold drawn, high frequency welded.2.Polishing:600#,400#, 320#, 240#, 180#.etc.3 ...