JIS G3445 STKM11A, STKM 12A, STKM 12C, STKM13A is the standard applicable to carbon steel tubes for machine structural purposes Application JIS G3445 STKM 11A, STKM 12A, STKM 12C, STKM 13A, STKM 13C, STKM 15A, STKM 15C, STKM 17A, STKM 17C carbon steel tubes used for machinery, automobiles, bicycles, furniture, appliances and other machine parts. Grade and Designation The grade and designation of the tube shall be as given as follows table. The sub classification letter symbols A, B and C are used to indicate distinction of method of manufacturing the tube, cold working process, heat treatment, etc. Grade Designation Grade 11 A STKM 11A Grade 12 A STKM 12A B STKM 12B C STKM 12C Grade 13 A STKM 13A B STKM 13B C STKM 13C Grade 14 A STKM 14A B STKM 14B C STKM 14C Grade 15 A STKM 15A C STKM 15C Grade 16 A STKM 16A C STKM 16C Grade 17 A STKM 17A C STKM 17C Grade 18 A STKM 18A B STKM 18B C STKM 18C Grade 19 A STKM 19A C STKM 19C Grade 20 A STKM 20A ...