Patent ID: 8876986
Filing Date: 2014-11-04
Classification: C21D,C22C,Y10T

Abstract:
1. A cold-rolled steel sheet: comprising: iron (Fe), carbon (C) in a content of 0.05 to 0.30 percent by mass, silicon (Si) in a content of 3.0 percent by mass or less, manganese (Mn) in a content of more than 0.1 percent by mass but 2.8 percent by mass or less, phosphorus (P) in a content of 0.1 percent by mass or less, sulfur (S) in a content of 0.005 percent by mass or less, nitrogen (N) in a content of 0.01 percent by mass or less, aluminum (A1) in a content of 0.01 to 0.50 percent by mass, and further comprising-vanadium (V) in a content of 0.001 to 1.00 percent by mass, or at least one element selected from the group consisting of niobium (Nb), titanium (Ti), and zirconium (Zr) in a total content of 0.01 percent by mass or more, if the at least one element is present, following Expression 1 is satisfied: where [% C], [% Nb], [% Ti], and [% Zr] represent contents of percent by mass of C, Nb, Ti, and Zr, respectively, wherein the cold-rolled steel sheet comprises tempered martensite in a content of 50 percent by area or more with a remainder comprising ferrite, the tempered martensite has a number density of precipitates having an equivalent circle diameter of 1 to 10 nm of 20 or more per 1 μm