C 15 was a grade listed in DIN 1652-2. The K indicates that it was cold drawn. This is an obsolete standard, which has been superceded by DIN EN 10277-2. The new grade is called C15 (1.0401). +C indicates that it is cold drawn. It is nominally equivalent to S15C in JIS standards or 1015 in SAE/ASTM standards.