US 7,507,307 B2
Method for producing cold rolled steel plate of super high strength
Kohei Hasegawa, Hiroshima (Japan); Nobuyuki Nakamura, Hiroshima (Japan); and Toshiaki Urabe, Chiba (Japan)
Assigned to JFE Steel Corporation, Tokyo (Japan)
Appl. No. 10/485,229
PCT Filed Jun. 06, 2003, PCT No. PCT/JP03/07215
§ 371(c)(1), (2), (4) Date Mar. 29, 2004,
PCT Pub. No. WO03/104499, PCT Pub. Date Dec. 18, 2003.
Claims priority of application No. 2002-168210 (JP), filed on Jun. 10, 2002.
Prior Publication US 2004/0177905 A1, Sep. 16, 2004
Int. Cl. C21D 8/02 (2006.01)
U.S. Cl. 148—603  [148/602; 148/661; 148/663; 148/652] 18 Claims
OG exemplary drawing
 
1. A method for manufacturing an ultra high strength cold-rolled steel sheet, said steel being a ferrite martensite dual-phase steel, the method comprising continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 1.4 to 2% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol.Al, 0.005% or less N, 0.0003 to 0.003% B, optionally at least one element selected from the group consisting of Ti and Mo, and the balance being Fe,
wherein said continuous annealing comprises the steps of:
(a) heating the cold-rolled steel sheet at from 800° C. to 870° C. for 10 seconds or more to provide a heated steel sheet;
(b) slowly cooling the heated steel sheet down to from 670° C. to 750° C. to provide a slowly cooled steel sheet;
(c) rapidly cooling the slowly cooled steel sheet down to 100° C. or less at a cooling speed of over 500° C./sec to provide a rapidly cooled steel sheet;
(d) reheating the rapidly cooled steel sheet at from 325° C. to 425° C. for from 5 minutes to 20 minutes to provide a reheated steel sheet;
(e) cooling the reheated steel sheet down to room temperature to provide a cooled steel sheet; and
(f) coiling the cooled steel sheet,
thereby obtaining a steel sheet having a tensile strength of 980 MPa or more, a hole-expanding ratio of 60% or more and an elongation of 15% or more.