High-tensile steel sheet and a method of manufacturing the same with excellent drop weight characteristics

摘要

PROBLEM TO BE SOLVED: To provide a thick steel plate having high strength of tensile strength: 780 MPa or more and excellent drop-weight characteristic of NDT temperature: -50°C or less without adding a special heat treatment to a steel plate for a direct hardening.;SOLUTION: A high tensile steel plate having a predetermined composition and an H value calculated by the formula (1) of 60 or more has a tempered bainite having an average particle diameter of old γ particles (dγ) of 60 μm or less as a main structure. H value=3.0×10⁴×0.1×(C*)^0.5×(0.7×[%Si]+1)×(3.33×[%Mn]+1)×(0.35×[%Cu]+1)×(0.36×[%Ni]+1)×(2.16×[%Cr]+1)×(3.0×[%Mo]+1)/T^1.5 (1), where C*=[%C]-1/4×([%Ti]-48×[%N]/14), T: steel plate thickness (mm), and [%A] represents the content of A element in the steel.;COPYRIGHT: (C)2014,JPO&INPIT