A fastening element has a core zone (14) formed of a relatively hard carbon steel, a skin zone (17) located outwardly of the core zone (14) and formed of a first low-carbon austenitic steel alloyed with a first alloy metal, and an intermediate zone (15a, 15b) provided between the core zone (14) and the skin zone (17) and formed of a second low-carbon steel having a smaller hardness than the steel the core zone (14) is formed of.