Importance

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Abstract

A Ni steel has a chemical composition within a predetermined range, in which a metallographic structure of a thickness middle portion contains 2.0 vol % to 20.0 vol % of an austenite phase, an average grain size of prior austenite grains is 3.0 μm to 12.0 μm, an average aspect ratio of the prior austenite grains is 2.6 to 10.0, a plate thickness is 4.5 mm to 20 mm, a yield stress at room temperature is 590 MPa to 710 MPa, and a tensile strength at the room temperature is 690 MPa to 810 MPa, when the plate thickness is more than 16 mm, the Ni steel contains Ni: 11.5% or more, and when the plate thickness is 16 mm or less and the Ni steel contains Ni: less than 11.5%, the average grain size of the prior austenite grains is 6.0 μm or less.

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First Claim