| 摘要 |
PROBLEM TO BE SOLVED: To provide a non-heattreated type low yield ratio high tensile strength steel plate free from the need of preheting in welding execution and having <=75% yield ratio and >=590 MPa tensile strength. SOLUTION: This steel plate is the one having a chemical compsn. contg. 0.02 to 0.04% C and 0.0002 to 0.002% solid solution B, and in which CEN expressed by the following formula satisfies the range of 0.21 to 0.30% and having a structure essentially consisting of bainite, and in which insular martensite of 0.8 to 2.5 vol.% is dispersed: CEN (%)=[C]+A (c) [Si]/24+[Mn]/6+[Cu]/15+[Ni]/20+([Cr]+[Mo]+[Nb]+[V])/5+5[B]}, where A (c)=0.75+0.25 tanh 20 ([C]-0.12)}, and [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Nb], [V] and [B] respectively denote the contents (mass %) of C, Si, Mn, Cu, Ni, Cr, Mo, Nb, V and B.
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