HIGH STRENGTH COLD-ROLLED STEEL SHEET 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 μm2 of ferrite, and the tensile strength is 980 MPa or more.

主权项

1. A high strength cold-rolled steel sheet exhibiting little variation in strength and ductility comprising:
C: 0.10-0.25% (means mass %, hereinafter the same with respect to the chemical composition); Si: 0.5-2.0%; Mn: 1.0-3.0%; P: 0.1% or less (exclusive of 0%); S: 0.01% or less (exclusive of 0%); Al: 0.01-0.05%; and N: 0.01% or less (exclusive of 0%) respectively, with the remainder consisting of iron and inevitable impurities, wherein a microstructure includes ferrite that is a soft first phase by 20-50% in terms of area ratio, with the remainder consisting of tempered martensite and/or tempered bainite that is a hard second phase; among all grains of the ferrite, a total area of grains that have an average grain size of 10-25 μm accounts for 80% or more of a total area of all grains of the ferrite; the dispersion state of cementite grains that have an equivalent circle diameter of 0.3 μm or more present in all grains of the ferrite is more than 0.15 piece and 1.0 piece or less per 1 μm2 of the ferrite; and the tensile strength is 980 MPa or more.