Patent ID: 7887924

Claim:
A high-strength steel having a surface compound layer thereon, wherein: (A) the high-strength steel has a tensile strength not lower than 1800 N/mm 2 and consists essentially of: C in an amount of 0.3 to 0.65 percent by mass; Cr in an amount of 0.95 to 5.0 percent by mass; Mn in an amount of 0.05 to 0.6 percent by mass; Si in an amount of 0.7 to 2.5 percent by mass; at least one of Mg, Ca, Sr, Ba, Li, Na and K so as to meet the following conditions: (1) the upper limit of the Mg, Ca, Sr, Ba, Li, Na and K content is 0.05% each, and (2) the Mg, Ca, Sr, Ba, Li, Na and K content meet expression (I) Cr + Mn 4 ≤ 1000 × [ Ca + Mg + Sr + Ba 2 + Li + Na + K 8 ] ( I ) where the respective Mg, Ca, Sr, Ba, Li, Na and K contents are in percent by mass of the high strength steel; and balance of iron and inevitable impurities; and (B) the surface compound layer comprises at least one compound of at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Li, Na and K, and occupies at least 20% of a unit length of a section of the steel in an observation field or above.