For the purpose of enhancing the fuel efficiency of an automobile and achieving collision safety, application of a high tensile strength steel sheet to a vehicle body frame structure is proceeding, but on the other hand, the increase in the strength of a material involves a decrease in formability of the material, and therefore, the steel sheet used may be required to satisfy both high press workability and high strength.
In a high-strength steel sheet, a retained (or residual) austenite steel having retained austenite in the steel structure may be known to, despite high strength, exhibit very high elongation by making use of a TRIP effect, In order to more increase the elongation of this retained austenite steel, for example, Patent Document 1 discloses a technique of ensuring uniform elongation by controlling two kinds of ferrite (bainitic ferrite and polygonal ferrite) while keeping the retained austenite fraction high.
Meanwhile, in forming a high-strength steel sheet having a tensile strength of 980 MPa or more, the work may be often performed mainly by bend forming but not draw forming that has prevailed in forming a low-strength steel sheet having a tensile strength of 440 MPa or less. Similarly to elongation, enhanced bendability may be required of also a high-strength sheet steel.
Conventionally, it has been known that V-bendability correlates with local ductility, and as a technique for enhancing the local ductility, Patent Document 1 discloses a technique of making the structure uniform and increasing the strength by adding a precipitation strengthening component to a ferrite single phase, and Patent Document 2 discloses a technique of similarly making the structure uniform by a structure mainly composed of bainite.
Also, Patent Document 3 discloses a high-strength high-ductility hot-dip galvanized steel sheet containing, in terms of volume fraction, from 30 to 90% of a ferrite phase, 5% or more of bainite, 10% or less of martensite, and from 5 to 30% of a retained austenite phase. Patent Document 4 discloses a high-strength cold-rolled steel sheet, where the density of dislocations contained in the steel sheet is 8×1011 (dislocations/mm2) or less, and the static/dynamic ratio (=FS2/FS1) as a ratio between a quasi-static strength (FS1) at a strain rate of 0.0067 (s−1), and a dynamic strength (FS2) at a strain rate of 1,000 (s−1) is 1.05 or more.
However, at present, higher strength and higher workability are required of also in the case of a high-strength steel sheet, and a technique capable of satisfying this requirement and also of satisfying both of the elongation and V-bendability at a sufficiently high level is not known.