摘要 |
<p>A high-strength steel plate characterized by containing, in terms of mass%, 0.18-0.23% carbon, 0.1-0.5% silicon, 1.0-2.0% manganese, up to 0.020% phosphorus, up to 0.010% sulfur, 0.5-3.0% nickel, 0.003-0.10% niobium, 0.05-0.15% aluminum, 0.0003-0.0030% boron, and up to 0.006% nitrogen, with the remainder being iron and incidental impurities, and having a composition which has an index of weld crack sensitivity (Pcm) calculated with Pcm = [C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B] of 0.36% or lower. The steel plate is further characterized by having an Ac3 transformation point of 830ºC or lower, martensite structure content of 90% or higher, yield strength of 1,300 MPa or higher, and tensile strength of 1,400-1,650 MPa. The steel plate is furthermore characterized in that the tensile strength [TS] (MPa) and the prior-austenite grain size number, N?, as calculated from the average number of crystal grains, m, per mm2 of a section of a sample piece using N?=-3+log2m, satisfy both N?=([TS]-1400)×0.004+8.0 and N?=11.0 when the tensile strength is lower than 1,550 MPa, and satisfy both N?=([TS]-1550)×0.008+8.6 and N?=11.0 when the tensile strength is not lower than 1,550 MPa.</p> |