| 摘要 |
PROBLEM TO BE SOLVED: To propose a high strength hot-rolled steel strip having a low yield ratio over the entire length by a composition system reduced in the content of Si which is the cause for the deterioration in the toughness of a resistance weld zone used in manufacturing a steel pipe or assembling components and more particularly the cause for the deterioration in the toughness of the weld zone used in large heat input arc welding. SOLUTION: The production method comprises subjecting a steel slab of≤0.4% Si in mass ratio to finish rolling in such a manner the cumulative draft until the surface temperature of the material to be rolled reaches (Ar<SB>3</SB>-30) from 920°C attains≥50%, then to primary cooling at a cooling rate of≥80°C/s down to a mean sectional temperature 700±30°C after the finish rolling; subjecting the steel sheet to a ferrite precipitation treatment of cooling the steel sheet for 6 to 15 s at a cooling rate of 0.5 to 5°C/s after the primary cooling; subjecting the steel sheet to secondary cooling down to 450°C at a cooling rate of≥10°C/s after the ferrite transformation treatment, thence coiling the steel sheet between 250 to 450°C. COPYRIGHT: (C)2006,JPO&NCIPI
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