Patent ID: 7485195

Claim:
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 Ar 3 to (Ar 3+ 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 T 0 determined by the chemical composition of the steel shown in the following relation (5) and a temperature of not more than 400° C.: TFE≧Ar 3 (° C.) (1) TFS≦1100° C. (2) Δε≧( TFS−TFE )/375 (3) 20° C.≦( TFS−TFE )≦120° C. (4) T 0 =−650.4×{C %/(1.82×C %−0.001)}+B (5) where, B is found from the composition of the steel expressed by weight %, B = - 50.6 × Mn ⁢ eq + 894.3 Mn ⁢ eq = Mn ⁢ ⁢ % + 0.24 × N ⁢ i ⁢ ⁢ % + 0.13 × Si ⁢ ⁢ % + 0.38 × Mo ⁢ ⁢ % + 0.55 × Cr ⁢ ⁢ % + 0.16 × Cu ⁢ ⁢ % - 0.50 × Al ⁢ ⁢ % - 0.45 × Co ⁢ ⁢ % + 0.90 × V ⁢ ⁢ % where , ⁢ Ar 3 = 901 - 325 × C ⁢ ⁢ % + 33 × Si ⁢ ⁢ % + 287 × P ⁢ ⁢ % + 40 × Al ⁢ ⁢ % - 92 × ( Mn ⁢ ⁢ % + Mo ⁢ ⁢ % + Cu ⁢ ⁢ % ) - 46 × ( Cr ⁢ ⁢ % + Ni ⁢ ⁢ % ) 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, Δε i=εi × exp{−(ti*/τ n ) 2/3 } τn= 8.46×10 −9 ×exp{43800/R/Ti} ti*=τ n× (ti/τ i+t ( i+ 1)/τ( i+ 1)+ . . . + tn/τn}.