High-strength cold-rolled steel sheet excellent in bending workability

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 contents will be simply expressed in “%”);
• 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 (S) 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 cm2 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” refers to a group of inclusions which includes an (n−1)-ary group of inclusions (wherein “n” is an integer of 1 or more;
• when “n” is 1, a “zero-ary group of inclusions” refers to an inclusion particle) and at least one neighboring x-ary group of inclusions (wherein “x” is an integer of from 0 to n−1, where “n” is an integer of 1 or more;
• a “zero-ary group of inclusions” refers to an inclusion particle), in which the minimum intersurface distance (γ) 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:
• γ≦(1.9−0.0015σy)×(d1+d2) (1-1)
• wherein
• γ 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;
• σy represents the yield strength (MPa) of the steel sheet;
• d1 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
• d2 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.