High-strength cold-rolled steel sheet and production method for same

A high strength cold rolled steel sheet having a composition including:

• by mass,
• C in an amount of not less than 0.04% but not more than 0.16%;
• Si in an amount of not less than 0.15% but not more than 1.25%;
• Mn in an amount of not less than 2.00% but not more than 3.50%;
• P in an amount of not more than 0.050%;
• S in an amount of not more than 0.0050%;
• N in an amount of not more than 0.0100%;
• Al in an amount of not less than 0.010% but not more than 2.000%;
• Ti in an amount of not less than 0.005% but not more than 0.075%;
• Nb in an amount of not less than 0.005% but not more than 0.075%;
• and
• B in an amount of not less than 0.0002% but not more than 0.0040%,
• with the balance being Fe and inevitable impurities, and
• a steel structure including:
• by volume fraction, ferrite of not less than 10% but not more than 70%;
• retained austenite of not less than 1% but not more than 10%;
• bainite of not less than 10% but not more than 60%;
• and martensite of not less than 2% but not more than 50%,
• wherein the ferrite has an average grain size of not more than 6.0 μm, the retained austenite has an average grain size of not more than 4.0 μm, the bainite has an average grain size of not more than 6.0 μm, and the martensite has an average grain size of not more than 4.0 μm, and
• wherein a concentration ratio of an average Si concentration in a region extending from a surface up to 10 μm in a depth direction in the high strength cold rolled steel sheet to an average Si concentration in a whole of the high strength cold rolled steel sheet is more than 1.00 but less than 1.30 by mass ratio.