| 摘要 |
PROBLEM TO BE SOLVED: To provide a high burring hot rolled steel sheet having >490 MPa class strength and baking hardenability and also to provide its manufacturing method. SOLUTION: The high burring hot rolled steel sheet with baking hardenability has a composition consisting of, by mass, 0.01 to 0.1% C, 0.005 to 2% Si, 0.1 to 3% Mn,≤0.1% P,≤0.03% S, 0.001 to 0.1% Al,≤0.005% N, 0.05 to 0.2% Ti and the balance Fe with inevitable impurities and further satisfying, with respect to the above C, S, N and Ti, Ti(*)=Ti-(48/14)N-(48/32)S≥0% and C-(12/48)Ti(*)≤0.05% and also has a structure where a microstructure is composed mainly of bainitic ferrite and the average grain size of old austenite grains ranges from >8 to 80μm. In the method for manufacturing the steel sheet, finish rolling of 1 to 15% draft at the final stage for a steel slab with the above composition is finished at≥Ar<SB>3</SB>+50°C and, after 0.5 to 5 s after it, the resultant steel sheet is cooled at (80 to 500)°C/s cooling rate through the temperature region between Ar<SB>3</SB>to Bf and then cooled at≥20°C/s cooling rate below to≤500°C. COPYRIGHT: (C)2005,JPO&NCIPI
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