Steel Plate

Abstract: This steel plate has a prescribed chemical composition. When the plate thickness is represented by t, the metal structure at the t/4 position, which is at t/4 from the surface in a plate-thickness-direction cross-section, contains at least 70% of martensite and at least 10% of residual austenite by volume. The maximum grain diameter of the residual austenite is below 5.0 µm. When the Mn concentration is measured at a plurality of measurement points at 1 µm intervals in a square region in which each side measures t/4, centered at the t/4 position, in a plate-thickness-direction cross-section, the proportion of measurement points at which the Mn concentration is at least 1.1 times the average value of the Mn concentration at all of the plurality of measurement points is less than 10.0%. The tensile strength is 1470 MPa or above.

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