| 摘要 |
PROBLEM TO BE SOLVED: To inexpensively produce an extremely thin hot rolled steel sheet with <1.2mm sheet thickness excellent in impact resistance at a high strain rate at high yield without deteriorating its bore expandability. SOLUTION: A steel stock contg., by weight, 0.0001 to 0.02% C, 0.01 to 2.0% Si, 0.1 to 3.0% Mn, 0.01 to 0.15% P, <=0.010% S, 0.001 to 0.05% Al, and the balance Fe with inevitable impurities is heated at 900 to 1250 deg.C and is subjected to rough rolling to form into a sheet bar. Next, this sheet bar is joined with the preceding sheet bar, and, while lubrication is executed under the conditions of >=900m/min rolling rate, the Ar3 transformation point to (the Ar3 transformation point + 50 deg.C) rolling finishing temp. and >=95% draft, it is subjected to finish rolling into <1.2mm sheet thickness. Within 0.5sec after that, it is cooled to <=450 deg.C at a cooling rate of >=50 deg.C/sec and is coiled round a coil to obtain the extremely thin hot rolled steel sheet in which the metallic structure is composed of a ferrite single phase one having >=12 ferrite grain size number and the thickness of the oxidized layer on the surface is regulated to <=3μm.
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