| 摘要 |
PURPOSE: To obtain excellent crack arrest characteristic and hydrogen induced cracking resistance by specifying the composition of a thick plate of low carbon steel containing Ti, Nb, Ca, etc., and also controlling the structure in the cross section of the steel plate or the crystalline grain size. CONSTITUTION: This steel plate has a composition containing, by weight ratio, 0.02-0.12% C, <=0.6% Si, 0.6-1.5% Mn, <=0.015% P, <=0.001% S, <=0.06% Al, 0.005-0.03% Ti, 0.01-0.10% Nb, 0.001-0.005% Ca, <=0.003% O, and 0.001-0.005% N and also satisfying the condition represented by an inequality. Moreover, this steel plate has >=25mm thickness and >=500MPa strength. Further, in the range from the surface and the back surface to the center positions each at a depth of 10-25% of plate thickness in a plate thickness direction, the area ratio of ferrite of <=5μm crystalline grain size and Vickers hardness are regulated to >=70% and >=140, respectively. The inner part of plate thickness, as the balance, is composed of accicular ferrite and has <=248 Vickers hardness.
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