HIGH STRENGTH COLD-ROLLED STEEL PLATE EXHIBITING LITTLE VARIATION IN STRENGTH AND DUCTILITY, AND MANUFACTURING METHOD FOR SAME

摘要

In a high strength cold-rolled steel plate having a specific chemical composition, a soft first phase (ferrite) has an area ratio of 20-50%, the remainder being a hard second phase (tempered martensite and/or tempered bainite), among all the ferrite grains, ferrite grains that have an average grain diameter of 10-25 µm account for a total area ratio of 80% or more, the number of the cementite grains that have an equivalent circle diameter of 0.3 µm or more is more than 0.15 piece and 1.0 piece or less per 1 µm 2 of ferrite, and the tensile strength is 980 MPa or more.