US 7,485,195 B2
High-strength hot-rolled steel sheet excellent in shape fixability and method of producing the same
Natsuko Sugiura, Futtsu (Japan); Manabu Takahashi, Futtsu (Japan); Naoki Yoshinaga, Kimitsu (Japan); and Ken Kimura, Futtsu (Japan)
Assigned to Nippon Steel Corporation, Tokyo (Japan); and Arcelor France, St. Denis (France)
Appl. No. 10/561,133
PCT Filed Jun. 28, 2004, PCT No. PCT/JP2004/009465
§ 371(c)(1), (2), (4) Date Dec. 15, 2005,
PCT Pub. No. WO2005/005670, PCT Pub. Date Jan. 20, 2005.
Claims priority of application No. 2003-182675 (JP), filed on Jun. 26, 2003; and application No. 2004-092280 (JP), filed on Mar. 26, 2004.
Prior Publication US 2007/0089814 A1, Apr. 26, 2007
Int. Cl. C22C 38/02 (2006.01); C22C 38/06 (2006.01); C22C 38/18 (2006.01); C22C 38/16 (2006.01); C22C 38/40 (2006.01); C21D 8/02 (2006.01)
U.S. Cl. 148—320  [148/330; 148/332; 148/333; 148/334; 148/335; 148/336; 148/602; 148/654; 148/331] 10 Claims
 
1. A method of producing a high-strength hot-rolled steel sheet excellent in shape fixability comprising the following steps,
hot-rolling a cast slab as cast or cooled once,
said cast slab containing, in terms of weight %,
C: 0.02 to 0.3%,
at least one or more element selected from the group consisting of, total 0.1 to 3.5%, in terms of weight %,
Mn: 0.05 to 3%,
Ni: 3% or less
Cr: 3% or less,
Cu: 3% or less,
Mo: 1% or less,
Co: 3% or less and
Sn: 0.2% or less,
at least one or both of, total 0.02 to 3% in terms of weight %,
Si: 3% or less and
Al: 3% or less
and remainder Fe and unavoidable impurities,
then reheated to a range of 1000 to 1300° C., with a total reduction ratios of 25% or more at Ar3 to (Ar3+150)° C., temperature at finishing hot-rolling start, TFS, and temperature at finishing hot-rolling end, TFE, and calculated residual strain Δε to simultaneously satisfy following relations (1) to (4), and
cooling hot-rolled steel sheet, then
coiling at below critical temperature T0 determined by the chemical composition of the steel shown in the following relation (5) and a temperature of not more than 400° C.:
TFE≧Ar3 (° C.)  (1)
TFS≤1100° C.  (2)
Δε≧(TFS−TFE)/375  (3)
20° C.≤(TFS−TFE)≤120° C.  (4)
T0=−650.4×{C %/(1.82×C %−0.001)}+B  (5)
where, B is found from the composition of the steel expressed by weight %,

OG Complex Work Unit Drawing
Δε is found from the equivalent strain εi (i is 1 to n) given at each stand of the n stages of finishing rolling for the rolling, time ti (sec) (i=1 to n−1) between stands, time tn (sec) from the final stand to the start of cooling, rolling temperature Ti(K) (i=1 to n) at each stand, and a constant R=1.987,
ε=Δε1+Δε2+ . . . +Δεn
where, Δεi=εi ×exp{−(ti*/τn)2/3}
τn=8.46×10−9×exp{43800/R/Ti}
ti*=τ(ti/τi+t (i+1)/τ(i+1)+ . . . +tn/τn}.