Patent ID: 6045630
Filing Date: 2000-04-04
Classification: C21D,C22C

Abstract:
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%;B: 0.0004% to 0.0025%;Al: not greater than 0.1%;Cu: 0% to 0.6%;Cr: 0% to 0.8%;Mo: 0% to 0.6%;V: 0% to 0.1%;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 0.4% to 0.58%: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.