| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a high strength steel plate of excellent delayed fracture resistance. <P>SOLUTION: The high strength steel plate of excellent delayed fracture resistance is characterized in that its chemical components (C, Si, Mn, Al, P, S) are each within a specified range with the balance being iron and inevitable impurities, martensite occupies at least 95 area% of the whole structure, the structure in the thickness direction of the steel plate from a position at a depth of 10 μm from the surface to a position at a depth of a quarter of the plate thickness satisfies formula (1) shown below, and the tensile strength is 1180 MPa or greater. Formula (1): 6.7×10<SP POS="POST">-3</SP>×Dγ+7.4×10<SP POS="POST">-9</SP>×ρ<SP POS="POST">1/2</SP>-0.073×Nc+0.092×p<SP POS="POST">2</SP>≤0.570. In formula (1), Dγ represents the old γ grain diameter (μm), ρ represents the dislocation density (m<SP POS="POST">-2</SP>), Nc represents the concentration of solid solution C present in the martensite (mass%), and p represents the ratio of the length of the carbide precipitated in the old γ grain boundaries relative to the length of the old γ boundaries (provided that 0≤p≤1). <P>COPYRIGHT: (C)2013,JPO&INPIT |
