| 摘要 |
PROBLEM TO BE SOLVED: To produce a rolled bar steel capable of providing at least >=1000 N/mm<2> bolt strength and improving strength and delayed fracture resistance, capable of performing primary wiredrawing without softening of bar steel, and also capable of carrying out bolt forming by cold forging. SOLUTION: Among components, Si content is minimized (<=0.05%) and N is perfectly fixed (0.0030-0.0100%, and further, Ti+Nb is regulated to 0.010-0.050% and also the value of solid solution N represented by (solid solution N)=[N-(14/48)Ti-(14/93)Nb}×10000 is regulated to -50 to 0 to improve cold forgeability and delayed fracture resistance). Moreover, Cr content and Mo content are regulated to rather high values of 1.0-2.0% and 0.20-1.0%, respectively, and the ideal critical diameter DI is regulated to 80-130 mm, and a steel billet capable of securing strength even in the case of tempering at high temperature is subjected to hot controlled rolling to form a microstructure composed of ferrite having <=20μm grain size and pearlite.
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