Abstract: A dual phase steel sheet with good bake-hardening properties is provided. The steel sheet is characterized in containing (in terms of percent by mass) C: no less than 0.06% and less than 0.25%; Si+Al: 0.5 to 3%; Mn: 0.5 to 3%; P: no more than 0.15%; and S: no more than 0.02%; and also meeting the following condition (in terms of space factor) that retained austenite is at least 3%, bainite is at least 30%, and ferrite is no more than 50%, and further characterized in differing in stress larger than 50 MPa before and after application of 2% pre-strain and ensuing heat treatment for paint baking at 170° C. for 20 minutes. The steel sheet has well-balanced strength and workability, exhibits good bake-hardening properties at the time of paint baking, and offers good resistance to natural aging.

Abstract: A high-strength steel sheet comprises carbon: 0.06 to 0.25 mass %, Si: 0.5 to 3.5 mass % and Mn: 0.7 to 4 mass %. Its mother structure is ferrite, its second phase structure comprises martensite and the residual austenite and the second phase structure measured by image analysis has an area fraction of 25 % or less based on the total structure. The steel sheet satisfies the following requirements (1) to (3): (1) the volume fraction (Vt&ggr;hd R) of the residual austenite is 5 % or more; (2) the ratio (SF&ggr;R/Vt&ggr;R) of the area fraction (SF&ggr;R) of the residual austenite within ferrite to Vt&ggr;R is 0.65 or more; and (3) the ratio [&agr;2/(&agr;1+&ggr;R)] of the space factor (&agr;2) of martensite to the second phase structure (&agr;1+&ggr;R) is 0.25 to 0.60. The steel sheet has excellent balance between strength and local elongation, and a low yield ratio.

Abstract: Disclosed is a dual-phase steel sheet having low yield ratio, excellent in the balance for strength-elongation and for strength-stretch flange formability, and also excellent in bake hardening property containing (on the mass% basis).

Abstract: A P- and Ti-added galvannealed steel sheet superior in ductility, and a process for production thereof. It is made of a cold-rolled steel sheet and has alloyed hot-dip galvanizing on the surface thereof, said cold-rolled steel sheet having the chemical composition (in terms of wt %) of C: less than 0.010%, Si: no more than 0.5%, Mn: 1.0˜3.0%, P: no more than 0.20%, S: no more than 0.01%, Al: 0.005˜0.10%, N: no more than 0.0050%, Ti/48−(C/12+N/14+S/32): 0.0003˜0.0018 with the remainder being chiefly Fe, and said cold-rolled steel sheet being characterized by &rgr;1≦107 and &rgr;2≦5×105, where &rgr;1 is the number of precipitates whose particle diameter (D) is in the range of 10 nm≦D<100 nm and &rgr;2 is the number of precipitates whose particle diameter (D) is in the range of 100 nm≦D.

Abstract: A hot-dip galvanized steel sheet which is produced from a cold-rolled steel sheet, as a base steel sheet, consisting essentially of C: 0.010-0.06 wt %, Si: no more than 0.5 wt %, Mn: no less than 0.5 wt % and less than 2.0 wt %, P: no more than 0.20 wt %, S: no more than 0.01 wt %, Al: 0.005-0.10 wt %, N: no more than 0.005 wt %, Cr: no more than 1.0 wt %, Mn+1.3Cr: 1.9-2.3 wt %, Fe: remainder, and having a structure composed of ferrite and a second phase containing martensite, said second phase in the structure accounting for no more than 20% in terms of area and martensite in the second phase accounting for no less than 50%, and which has a zinc-plated layer formed on the surface thereof by hot-dip galvanizing or hot-dip galvannealing. A process for production of said hot-dip galvanized steel sheet. This steel sheet has a composite structure containing martensite and yet it has a low strength (no higher than 500 MPa) and also has good strength-ductility balance.