Unfortunately the grain size you have just calculated is not achievable by commercial plate rolling practice. We can achieve a grain size of ˜ 7 µm using commercial rolling schedules but the short fall in yield strength will be made up using the other strengthening mechanisms that you have identified. The final composition of the steel selected is that of a high strength low alloy (HSLA) steel:

0.1 C, 0.3 Si, 1.5 Mn, 0.01 Nb, 0.05 V and no free N all in wt%

As you can see we have a lower carbon level than the maximum allowed in the specification (this will give us better weldability through a lower carbon equivalent). We also have microalloying elements present, i.e. Nb and V, which provides the precipitation strengthening required to achieve the desired yield strength.