A method of manufacturing the same, as well as high-strength hot-dip galvanized steel sheet and high-strength cold-rolled steel sheet having excellent ductility balance - strength and deep drawability

摘要

PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet excellent in deep-drawability and strength-ductility balance.;SOLUTION: To a hot-rolled coil having composition composed by mass% of 0.05-0.2% C, 0.1-2.0% Si, 0.005-1.5% Al, and the suitable contents of Mn, P, S, N and adjusting to 0.5-2.5% Si+Al and having bainite and martensite as the main structure: a batch hot-rolled sheet annealing process for holding the coil in the range of 550°C to Ac₁ point for ≥1 hr, a cold-rolling process and a recrystallizing annealing process, in which a continuous recrystallizing annealing for heating the cold-rolled sheet to Ac₁ to Ac₃ temperature at ≥5°C/s average temperature-raising speed in the Ac₁ point to (Ac₁ point+50°C) and holding it for ≥5s, and successively, austemper-treatment for cooling the sheet to 350-500°C at ≥5°C/s cooling speed and holding the sheet at this temperature for 10-600s, are applied, are applied in this order. In this way, the high strength cold-rolled steel sheet having dual structure composed of the ferrite having ≥70% volume ratio as the main phase and a second phase having ≤3 μm average grain diameter containing a retained austenite having at least ≥3% volume ratio and ≥1.1 r-value and ≥22,000 MPa % strength-ductility balance TS×EL, is obtained.;COPYRIGHT: (C)2009,JPO&INPIT