High-strength steel sheet and manufacturing method therefor

A high-strength steel sheet having a component composition containing, in mass %,

• C:
• 0.07 to 0.14%,
• Si:
• 0.65 to 1.65%,
• Mn:
• 1.8 to 2.6%,
• P:
• 0.05% or less,
• S:
• 0.005% or less,
• Al:
• 0.08% or less,
• N:
• 0.0060% or less,
• Ti:
• 0.005 to 0.030%,
• B:
• 0.0002 to 0.0030%, and
• either or both of Cr:
• 0.01 to 0.40% and Mo:
• 0.01 to 0.50%, with the balance being Fe and incidental impurities, and Expression (1) being satisfied, wherein:
• an average grain size of a ferrite phase is 1.5 μm or less;
• an area ratio of the ferrite phase is 2% or more and 15% or less;
• an area ratio of a tempered martensite phase is 75% or more and 96% or less;
• and a total length per unit area of an interface between an untempered martensite phase and the ferrite phase and an interface between the untempered martensite phase and the tempered martensite phase is 6.3×108 μm/m2 or more and 5.0×1011 μm/m2 or less;
• [
• %
• 
• 
• Cr
• ]
• +
• 2.6
• 
• [
• %
• 
• 
• Mo
• ]
• [
• %
• 
• 
• C
• ]
• ≥
• 2.0
• (
• 1
• )
• wherein [% M] (M=Cr, Mo, C) represents content of each element in mass %.