| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is excellent in deep drawability and useful for a steel sheet for automobile or the like, the tensile strength (TS) of which is high strength of≥540 MPa, which has a high r value of≥1.2 and the in-plane anisotropy of r value of which is small to be ¾Δγ¾≤0.20. SOLUTION: A steel slab, which is prepared specially so that C and Nb satisfy 0.035-0.05% C, 0.05-0.12% Nb and (Nb/93)/(C/12)=0.15-0.45, is heated at the temperature of≥1,000 to≤1,200°C and at the temperature satisfying SRT (slab heating temperature)≤25,000[%C][%Nb]+1,050, is subjected to a finish-rolling at≥800°C (finish rolling outlet side temperature), is coiled at 600-720°C and successively, is pickled and cold-rolled, is heated at the average temperature-raising rate of≥15°C/s up to 700°C, is heated and annealed at the average temperature-raising rate of 0.1-2°C/s from 700°C up to the annealing temperature of≥800 to≤950°C, and the is cooled to 500°C at the average cooling rate of≥5°C/s. COPYRIGHT: (C)2008,JPO&INPIT
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