DIN 2391 is an obsolete standard for seamless steel tubes. It has been replaced by DIN EN 10305-1. The ASTM equivalent standard for seamless steel tubes is ASTM A 519. Grade St 35 (1.0308), now called E235 in DIN EN 10305-1, is equivalent to grade 1015 in ASTM A 519. The BK symbol means that the tube is supplied in the cold drawn condition without any subsequent thermal treatment, so it is the highest strength condition for this grade. The ASTM terminology is "cold worked".
