High-toughness, high-tensile-strength steel and method of manufacturing the same

A high-tensile-strength steel with a tensile strength of not less than 900 MPa, consisting essentially of, by weight percent:

• C:
• 0.02% to 0.1%;
• Si:
• not greater than 0.6%;
• Mn:
• 0.2% to 2.5%;
• Ni:
• greater than 1.2% but not greater than 2.5%;
• Nb:
• 0.01% to 0.1%;
• Ti:
• 0.005% to 0.03%;
• N:
• 0.001% to 0.006%;
• Al:
• not greater than 0.1%;
• B:
• 0% to 0.0004%;
• Cu:
• 0% to 0.4%;
• Cr:
• 0% to 0.8%;
• Mo:
• 0% to 0.6%;
• V:
• 0% to 0.1%;
• and
• Ca:
• 0% to 0.006%;
• and balance Fe and incidental impurities;
• wherein the condition (a) and (b) below is satisfied, and P and S among unavoidable impurities are contained in an amount of not greater than 0.015% and not greater than 0.003%, respectively:
• (a):
• the carbon equivalent value Ceq defined by equation 1) below being 0.53% to 0.7%;
• Ceq=C+(Mn/6)+{(Cu+Ni)/15}+{(Cr+Mo+V)/5} 1):
• wherein each atomic symbol represents the content (wt. %) of the corresponding element;
• (b):
• a mixed structure of martensite and lower bainite occupying at least 90 vol. % in the microstructure;
• lower bainite occupying at least 2 vol. % in the mixed structure;
• and the aspect ratio of prior austenite grains being not less than 3.