To provide a thick steel plate having excellent brittle crack arrest properties which satisfies high strength (tensile strength of ≥490 MPa) and also satisfies ≥5,000 N/mm
In the thick steel plate, a chemical component composition is suitably regulated, further, in a microstructure at the position of t/4 to t/2 depth from the surface (t denotes plate thickness and so forth), the average area ratio of pseudo polygonal ferrite is 30 to 85%, also, at the position of the depth t/4 from the surface, provided that the grain size of the crystal grains surrounded by high angle grain boundaries in which the orientation difference between the adjoining two crystals is ≥15° is defined as the effective crystal grin diameter D (μm), this satisfies formula (1):-215+1.56×D+9.79×d+0.24×YS<-60 in the relation between the average circle-equivalent diameter d (μm) of island martensite and the yield stress YS (MPa) of the steel plate.
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TAMURA EIICHI
Tadashi Sugakawa
Osamu Futakuchi
Hiroaki Ito
Hisahiko Ueki