| 摘要 |
PROBLEM TO BE SOLVED: To improve the spot weldability of a steel rod and to balance its uniform elongation and strength in a high degree by subjecting rod steel obtd. by rolling a steel having a specified compsn. contg. C, Si, Mn, Al and N to quenching and tempering treatment at a specified temp. SOLUTION: A steel contains, by mass, 0.20 to 0.45% C, 0.5 to 2.0% Si, 0.5 to 3.0% Mn, 0.005 to 0.050% Al and 0.0040 to 0.0250% N. Rod steel obtd. by subjecting the steel whose componental compsn. is controlled thereto is subjected to ordinary quenching and is thereafter subjected to tempering treatment. In the case the tempering temp. is less than 100 deg.C, the microstructure of the steel lies in the state of martensite as-quenched, and stable ductility can not be obtd. In the tempering temp. region of >300 deg.C, the balance between the strength and uniform elongation is broken, and the object value of strength and uniform elongation can not be obtd. Moreover, the heating temp. at the time of the quenching is preferably controlled to the Ac3 to <=1,200 deg.C.
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