(57) The high strength steel plate contains C: 0.1-0.25% (means mass%, hereinafter the
same with respect to the chemical componential composition), Si: 0.1-0.5%, Mn: 0.5-2.0%,
Cr: 0.1-1.5%, Mo: 0.1-0.5%, Ti: 0.01-0.05%, and Nb: 0.01-0.05% respectively, and contains
V: 0.01-0.05% and/or B: 0.0001-0.005%, and the balance comprises iron with inevitable
impurities, wherein the average grain size of prior austenite is 20 µm or below, the
standard deviation (σ) of prior austenite grain size distribution is 5 µm or below,
and the tensile strength is 980 MPa or above. With such constitution, high strength
can be maintained while bending workability becomes excellent as well.
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