Steel plate and method of producing same

A steel plate comprising a chemical composition containing, in mass %,

• C:
• 0.02% or more and 0.15% or less,
• Si:
• 0.01% or more and 0.50% or less,
• Mn:
• 0.05% or more and 2.50% or less,
• Ni:
• 0.5% or more and less than 5.0%,
• P:
• 0.03% or less,
• S:
• 0.005% or less, and
• N:
• 0.0010% or more and 0.0080% or less,
• with a balance consisting of Fe and inevitable impurities,
• wherein in a microstructure at a depth position of ¼ of a plate thickness from a surface of the steel plate in a plate thickness direction, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250/mm2 or less.