Patent ID: 8460800

Claim:
A cold-rolled steel sheet comprising a steel having a composition of: a carbon (C) content of from 0.05 percent by mass to 0.3 percent by mass (hereinafter “%” means % by mass); a silicon (Si) content of 3.0% or less: a manganese (Mn) content of from 1.5% to 3.5%; a phosphorus (P) content of 0.1% or less; a sulfur (A) content of 0.05% or less; and an aluminum (Al) content of 0.15% or less, with the remainder including iron and inevitable impurities, wherein the steel has a microstructure comprising a composite structure including a ferrite structure and a martensite-containing second phase, and wherein, in a surface region from a surface to a depth one-tenth the gage of the steel sheet, the number density of n-ary groups of inclusions is 120 or less per 100 cm 2 of a rolling plane, in which each of the n-ary groups of inclusions is determined by an n-th Determination mentioned below, and, in each of the n-ary groups of inclusions, the distance in a steel sheet rolling direction between outermost surfaces of two outermost particles of the n-ary group of inclusions is 80 μm or more: n-th Determination the “n-ary group of inclusions” is a group of inclusions which includes (1) an inclusion selected from an (n−1)-ary group of inclusions (wherein “n” is an integer of 1 or more and (2) at least one inclusion selected from neighboring x-ary group of inclusions and inclusion particles where “x” is an integer from 0 to n−1, in which the minimum intersurface distance (λ) of nearest neighbor particles between the (n−1)-ary group of inclusions and the x-ary group of inclusions satisfies a condition represented by following Expression (1-1) and is 60 μm or less: λ≦(1.9−0.0015σ y )×( d 1 +d 2 ) (1-1) wherein λ represents the minimum intersurface distance (μm) of nearest neighbor particles between the (n−1)-ary group of inclusions and the x-ary group of inclusions; σ y represents the yield strength (MPa) of the steel sheet; d 1 represents the particle size (μm), in a steel sheet rolling direction, of the (n−1)-ary group of inclusions when “n” is 1, or represents the distance (μm), in a steel sheet rolling direction, between outermost surfaces of two outermost particles of the (n−1)-ary group of inclusions when “n” is 2 or more; and d 2 represents the particle size (μm), in a steel sheet rolling direction, of the x-ary group of inclusions when “x” is 0, or represents the distance (μm), in a steel sheet rolling direction, between outermost surfaces of two outermost particles of the x-ary group of inclusions when “x” is 1 or more.