High-strength steel plate having excellent formability, toughness and weldability, and production method of same

A steel sheet comprising a chemical composition comprising:

• by mass %,
• C in a range from 0.05 to 0.30%;
• Si of 2.50% or less;
• Mn in a range from 0.50 to 3.50%;
• P of 0.100% or less;
• S of 0.0100% or less;
• Al in a range from 0.001 to 2.000%;
• N of 0.0150% or less;
• O of 0.0050% or less;
• and
• the balance consisting of Fe and inevitable impurities,
• the steel sheet comprising a microstructure in a region from ⅛t to ⅜ t from a steel sheet surface, the microstructure comprising:
• by volume %,
• acicular ferrite of 20% or more;
• martensite of 10% or more;
• polygonal ferrite of 20% or less;
• residual austenite of 2.0% or less;
• and
• 5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure, wherein
• the martensite satisfies a formula (A) below,
• ∑
• i
• =
• 1
• 5
• d
• i
• a
• i
• 1.5
• ≤
• 10.
• (
• A
• )
• where:
• t represents sheet thickness, di represents a circle-equivalent diameter in μm of the i-th largest island-shaped martensite in the microstructure in the region of ⅛ t to ⅜ t, and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛ t to ⅜ t.