High-strength steel sheet having improved resistance to fracture and to HIC

1. A steel sheet having resistance to fracture and resistance to hydrogen-induced cracking characterized by having:

a chemical composition consisting of, in mass %, C: at least 0.02% to at most 0.04%, Si: at least 0.05% to at most 0.50%, Mn: at least 1.10% to at most 1.60%, P: at most 0.015%, S: at most 0.0030%, Nb: at least 0.005% to at most 0.030%, Ti: at least 0.005% to at most 0.020%, Al: at least 0.005% to at most 0.060%, Ca: at least 0.0005% to at most 0.0060%, N: at least 0.0015% to at most 0.0070%, at least one element selected from Cu, Ni, Cr, and Mo, the at least one element or more than one element being in a total amount of greater than 0.1% to less than 1.5%, and a remainder of Fe and impurities,

a steel structure comprising at least 10% by area to at most 16.5% of bainite and a remainder of ferrite and pearlite,

a degree of segregation of Nb which is less than 1.60 and a degree of segregation of Mn which is less than 1.40 both at the center of the thickness of the steel sheet,

a cracking area ratio of at most 5.0% when measured after immersion for 96 hours in an aqueous acetic acid solution at 25° C. with a pH of 4 which contains 5% sodium chloride and has a H 2 S partial pressure for saturation P H2S of 0.01×10 5 Pa, and

a percent ductile fracture of at least 85% in a DWTT test carried out at −30° C. DWTT-SA@-30 on a steel sheet with a thickness of at least 6 mm to at most 40 mm.

2. A steel sheet having resistance to fracture and resistance to hydrogen-induced cracking characterized by having:

a chemical composition consisting of, in mass %, C: at least 0.02% to at most 0.04%, Si: at least 0.05% to at most 0.50%, Mn: at least 1.10% to at most 1.60%, P: at most 0.015%, S: at most 0.0030%, Nb: at least 0.005% to at most 0.030%, Ti: at least 0.005% to at most 0.020%, Al: at least 0.005% to at most 0.060%, Ca: at least 0.0005% to at most 0.0060%, N: at least 0.0015% to at most 0.0070%, V: at most 0.10%, at least one element selected from Cu, Ni, Cr, and Mo, the at least one element or more than one element being in a total amount of greater than 0.1% to less than 1.5%, and a remainder of Fe and impurities,

a steel structure comprising at least 10% by area to at most 16.5% of bainite and a remainder of ferrite and pearlite,

a degree of segregation of Nb which is less than 1.60 and a degree of segregation of Mn which is less than 1.40 both at the center of the thickness of the steel sheet,

a cracking area ratio of at most 5.0% when measured after immersion for 96 hours in an aqueous acetic acid solution at 25° C. with a pH of 4 which contains 5% sodium chloride and has a H 2 S partial pressure for saturation P H2S of 0.01×10 5 Pa, and

a percent ductile fracture of at least 85% in a DWTT test carried out at −30° C. DWTT-SA@-30 on a steel sheet with a thickness of at least 6 mm to at most 40 mm.

3. A method of manufacturing a steel sheet characterized by:

heating a slab having a chemical composition as set forth in claim 1 to a heating temperature in T° C. which satisfies the following Equation (i) in a heating time which is at least 180 minutes and at most 480 minutes,

subjecting the heated slab to hot rolling in which finish rolling is completed at a temperature in the range of at least the Ar 3 point −60° C. to at most the Ar 3 point, where the Ar 3 point in ° C. is calculated by the following Equation (ii) to obtain a steel sheet having a dual phase structure, and

immediately cooling the resulting steel sheet at a cooling rate of at least 10° C. per second to a temperature in the range of 400-600° C. at which cooling is terminated:

6770/(2.26−log [Nb][C])−73> T≧ 6770/(2.26−log [Nb][C])−273  (i)

Ar 3 =910−310[C]−80[Mn]−20[Cu]−15[Cr]−55[Ni]−80[Mo]+0.35( t −8)   (ii)

wherein in above Equations (i) and (ii), the symbols for elements indicate the content in mass % of those elements, and in Equation (ii), t indicates the thickness of the steel sheet in mm after the completion of finish rolling.