High-strength steel sheet having excellent formability and low-temperature toughness, and method for producing same

1:

• A high-strength steel sheet having excellent formability and low-temperature toughness and consisting of, in mass %:
• C:
• 0.10 to 0.5%;
• Si:
• 1.0 to 3%;
• Mn:
• 1.5 to 3.0%;
• Al:
• 0.005 to 1.0%;
• P:
• more than 0% and not more than 0.1%;
• and
• S:
• more than 0% and not more than 0.05%;
• with the balance being iron and inevitable impurities,
• wherein a metal structure of the steel sheet contains polygonal ferrite, bainite, tempered martensite and retained austenite,
• and satisfying:
• (1) when the metal structure is observed by a scanning electron microscope,
• (1a) an area percent a of the polygonal ferrite to the entire metal structure is higher than 50%;
• (1b) the bainite is composed of a composite structure of high-temperature region generated bainite in which an average interval of distances between center positions of adjacent retained austenite grains, of adjacent carbide grains and of adjacent retained austenite grains and carbide grains is 1 μm or more and low-temperature region generated bainite in which an average interval of distances between center positions of adjacent retained austenite grains, of adjacent carbide grains and of adjacent retained austenite grains and carbide grains is less than 1 μm,
• wherein an area percent b of the high-temperature region generated bainite to the entire metal structure is 5 to 40%, and
• a total area percent c of the low-temperature region generated bainite and the tempered martensite to the entire metal structure is 5 to 40%;
• (2) a volume percent of the retained austenite measured by a saturation magnetization method to the entire metal structure is 5% or higher;
• (3) when an area enclosed by a boundary in which a crystal orientation difference measured by electron backscatter diffraction (EBSD) is 3° or larger is defined as a crystal grain, a distribution using each average IQ (Image Quality) based on the visibility of an EBSD pattern of the crystal grain analyzed for each crystal grain having a body centered cubic lattice (including a body centered tetragonal lattice) satisfies Equations (1) and (2) below:
• (IQave−IQmin)/(IQmax−IQmin)≧0.40 (1)
• (σIQ)/(IQmax−IQmin)≦0.25 (2)
• wherein IQave denotes an average value of average IQ total data of each crystal grain,
• IQmin denotes a minimum value of average IQ total data of each crystal grain,
• IQmax denotes a maximum value of average IQ total data of each crystal grain, and
• σIQ denotes a standard deviation of the average IQ total data of each crystal grain.