Patent ID: 8070887

Claim:
A steel plate having tensile strength in the width direction of 517 MPa to 990 MPa, the steel plate comprising, in its chemical composition by mass, C: 0.03 to 0.12%, Si: 0.8% or less, Mn: 0.8 to 2.5%, P: 0.03% or less, S: 0.01% or less, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.03%, Al: 0.1% or less, and N: 0.008% or less, so as to satisfy the expression Tiâˆ’3.4N>=0, and one or more of: Ni: 1% or less, Mo: 0.6% or less, Cr: 1% or less, Cu: 1% or less, V: 0.1% or less, Ca: 0.01% or less, REM: 0.02% or less, and Mg: 0.006% or less, and the balance being Fe and unavoidable impurities, wherein the steel plate has a high degree of a deformability and comprises a low-temperature transformation structure having a ferrite phase which is composed of first grains and a bainite phase which is composed of second grains, the ferrite phase being finely dispersed and accounting for 5 to 40% in area percentage of the structure, wherein the percentage of the ferrite grains larger than the average size of bainite grains is 10% or less in the ferrite phase, wherein the steel plate is produced by a process comprising the steps of: (a) reheating the steel slab containing the above defined steel compositions to an austenite temperature in the range of 1050 to 1250Â° C.; (b) after step (a), rough rolling the reheated steel slab within a recrystallization temperature range; (c) after step (b), finish rolling the rough rolled steel slab to a steel plate at a cumulative reduction ratio of at least 50% within a non-recrystallization temperature range of at most 900Â° C.; (d) lightly accelerated cooling the finish rolled steel plate at a first cooling rate of 5Â° C./sec to 20Â° C./sec from a temperature that is not lower than an Ar3 transformation point to a temperature in the range of 500Â° C. to 600Â° C.; and (e) immediately after step (d), or after maintaining the hot rolled steel plate at a constant temperature, or letting the hot rolled steel plate cool in air for at most 30 seconds, then heavily accelerated cooling the steel plate at a second cooling rate of at least 15Â° C./sec that is greater than the first cooling rate to a temperature less than 300Â° C.