Patent ID: 8827615

Claim:
A nut comprising: a thread portion having a female thread; a metallic plate portion having a base segment; and a hardness gradient portion provided between the thread portion and the metallic plate portion, wherein the thread portion, metallic plate portion and the hardness gradient portion are monolithic each other, a metallographic structure of the metallic plate portion differs from a metallographic structure of the thread portion, a hardness of the hardness gradient portion is lower than a hardness of the thread portion and lowers from the thread portion toward the metallic plate portion, the thread portion contains bainite as a main component, the base segment contains cementite and ferrite as a main component, the hardness gradient portion contains cementite and ferrite as a main component, and a metallographic structure of the ferrite in the hardness gradient portion becomes more microscopic from the thread portion to the metallic plate portion.