HIGH STRENGTH COLD ROLLED STEEL SHEET AND HIGH STRENGTH HOT-DIP GALVANIZED STEEL SHEET HAVING EXCELLENT DUCTILITY AND BENDABILITY, AND METHODS FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet which has a tensile strength of 980 MPa or greater and which exhibits excellent ductility and bendability.SOLUTION: There is provided the high strength cold rolled steel sheet which has a tensile strength of 980 MPa or greater and exhibits excellent ductility and bendability, and satisfies a specified component composition, and in which structure at a depth corresponding to a quarter of the sheet thickness satisfies all of (1) to (5) below when measured using specified methods. (1) The areal ratio of ferrite relative to the overall structure is 5% or more but less than 50%, with the remainder being hard phases. (2) The areal ratio of a mixed structure of fresh martensite and residual austenite relative to the overall structure is greater than 0% but 30% or less (3) A region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is present at 5% by area or greater. (4) When the fraction of a region in which the concentration of Mn is enriched to 1.2 times or more the concentration of Mn in the steel sheet is measured for 100 sections in a section of 2 μm square, the standard deviation is 4.0% or greater. (5) The concentration of Mn in a ferrite phase is 0.9 times or less the concentration of Mn in the steel sheet.