according to ISO 898, part 2
1) Nuts of these strength classes may be made from free cutting steel, unless other arrangements have been agreed upon between the buyer and the supplier. When using free cutting steel, the following maximum proportions of sulphur, phosphorus and lead are permitted:
sulfur 0,34 %
phosphorus 0,11 %
lead 0,35 %
2) For these strength classes it may be necessary to add alloys in order to achieve the mechanical properties of the nuts.