Patent ID: 8084143

Claim:
High yield ratio high-strength cold-rolled steel sheet superior in spot weldability and ductility, containing, by mass %, C: over 0.030 to less than 0.10%, Mn: 1.7 to 2.49%, P: 0.001 to 0.02%, S: 0.0001 to 0.006%, Al: 0.060% or less, N: 0.0001 to 0.0070%, Si: 0.54 to 0.65% Ti: 0.01 to 0.055%, Nb: 0.012 to 0.055%, Mo: 0.07 to 0.55%, B: 0.0005 to 0.0040%, and simultaneously satisfying 1.1<14 ×Ti (%)+20 ×Nb (%)+3 ×Mo (%)+300 ×B (%)≦3.7, the balance comprised of iron and unavoidable impurities, and having a microstructure composed of lower bainite or bainitic ferrite as a main phase which constitutes over 85% of the area of the microstructure, and having a yield ratio of more than 0.64 to less than 0.90, a TS×(E1) of 1/2 of 3320 or more, an YR×TS×(E1) 1/2 of 2320 or more, and a maximum tensile strength (TS) of 780 MPa or more, having a minimum value of CTS (cross-tensile strength) of 0.8 or more among 10 values of CTS obtained by welding 10 test pieces by a welding current of (CE+1.5)kA, wherein CE is welding current immediately before expulsion and surface flash, and wherein the minimum value of CTS among 10 values of obtained by welding 10 test pieces by a welding current of CE is defined as “1”, and having an X-ray intensity ratio of a {110} plane parallel to the sheet surface at ⅛ the thickness of the steel sheet of less than 1.0.