The invention discloses a super-thick low
alloy high strength Q345C steel plate which comprises the following chemical components by
mass (unit: wt %): C 0.08 to 0.18, Si 0.20 to 0.40, Mn 1.15 to 1. 50, P <=0.018, S <=0.005, micro-alloying elements of (Nb+Ni+Cr) <=0.020 and Als <= 0.040, with the rest being Fe and residual elements. Carbon equivalent satisfies the following equation: [Ceq=C+Mn / 5+(Cr+Mo+V) / 5+(Ni+Cu) / 15]<=0.43. According to the invention, total grade level of a variety of impurities is enabled
not to exceed 2.5 by carrying out a series of processes of converter
smelting, LF refining, vacuum refining,
casting, heating, rolling, controlled cooling,
slow cooling and heat treatment, designing the
chemical composition of multielement composite micro-
alloy elements and guaranteeing the cleanliness of steel by using the technology of LF+VD, and the grain size of steel is enabled to reach grade 8.0-10.0 through the
processing of controlled rolling, controlled cooling and normalization; effective implementation of above-mentioned measures enables the super-thick low
alloy high strength Q345C steel plate to be successfully manufactured.