Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High strength steel sheet and method for manufacturing the same

Carbide-free bainite in medium carbon steel

A high strength pressed member has excellent ductility and stretch flangeability and tensile strength of 780-1400 MPa, with a predetermined steel composition and steel microstructure relative to the entire microstructure of steel sheet, where area ratio of martensite 5-70%, area ratio of retained austenite 5-40%, area ratio of bainitic ferrite in upper bainite 5% or more, and total thereof is 40% or more, 25% or more of martensite is tempered martensite, polygonal ferrite area ratio is above 10% and below 50% to the entire microstructure of steel sheet, and average grain size is 8 μm or less, average diameter of a group of polygonal ferrite grains is 15 μm or less, the group of polygonal ferrite grains represented by a group of ferrite grains of adjacent polygonal ferrite grains, and average carbon content in retained austenite is 0.70 mass % or more and tensile strength is 780 MPa or more.

High-strength steel sheet and method for manufacturing same

Effect of austenite grain size on the transformation kinetics of upper and lower bainite in a low-alloy steel

Low temperature kinetics of bainite formation in high carbon steels

The present invention provides: a high-strength high-ductility hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having high fatigue resistance and corrosion resistance; a high-strength hot-dip galvanized steel sheet excellent in ductility, which improves non-plating defects and plating adhesion after severe deformation, and a method of producing the same; a high-strength and high-ductility hot-dip galvanized steel sheet having high fatigue resistance and corrosion resistance; a high-strength hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having superior appearance and workability, which suppresses the generation of non-plating defects, and a method of producing the same; and a high-strength hot-dip galvannealed steel sheet and a high-strength hot-dip galvanized steel sheet, which suppress non-plating defects and surface defects and have both corrosion resistance, in particular corrosion resistance in an environment containing chlorine ion, and high ductility, and a method of producing the same.

High-strength hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having fatigue resistance, corrosion resistance, ductility and plating adhesion, after severe deformation, and a method of producing the same

https://api.lib.kyushu-u.ac.jp/opac_download_md/1794503/Damage micromechanisms in dual-phase steel investigated with combined phase- and absorption-c.pdf

Damage micromechanisms in dual-phase steel investigated with combined phase- and absorption-contrast tomography

Bainite is of considerable importance in the design of high strength steels. There are two types of morphologies, upper and lower bainite. In upper bainite, cementite forms between adjacent bainitic ferrite plates. In certain steels, however, the cementite reaction is suppressed so that carbon-enriched austenite remains untransformed between bainitic ferrite plates. In lower bainite, cementite also has the opportunity to precipitate within bainitic ferrite plates. In order to model the development of these microstructures, it is necessary to treat the simultaneous formation of both the ferritic and carbide components of the microstructure. A theory has been developed to do exactly this, enabling the estimation of the phase fractions, the cementite particle size and the transition from upper to lower bainite. The results have been compared against experimental data.

Modelling Upper and Lower Bainite Trasformation in Steels

This cold-rolled steel sheet includes, in terms of mass %, C: not less than 0.05% and not more than 0.095%, Cr: not less than 0.15% and not more than 2.0%, B: not less than 0.0003% and not more than 0.01%, Si: not less than 0.3% and not more than 2.0%, Mn: not less than 1.7% and not more than 2.6%, Ti: not less than 0.005% and not more than 0.14%, P: not more than 0.03%, S: not more than 0.01%, Al: not more than 0.1%, N: less than 0.005%, O: not less than 0.0005% and not more than 0.005%, and contains as the remainder, iron and unavoidable impurities, wherein the microstructure of the steel sheet includes mainly polygonal ferrite having a crystal grain size of not more than 4 µm, and hard microstructures of bainite and martensite, the block size of the martensite is not more than 0.9 µm, the Cr content within the martensite is 1.1 to 1.5 times the Cr content within the polygonal ferrite, and the tensile strength is at least 880 MPa.

High-strength galvanized steel sheet, high-strength alloyed hot-dip galvanized sheet, and high-strength cold-rolled steel sheet which excel in moldability and weldability, and manufacturing method for the same

Provided is a spot welded joint ( 10 ) which includes at least one thin steel plate with a tensile strength of 750 MPa to 1850 MPa and a carbon equivalent Ceq of equal to or more than 0.22 mass % to 0.55 mass % and in which a nugget ( 3 ) is formed in an interface of the thin steel plates ( 1 A, 1 B). In a nugget outer layer zone, a microstructure consists of a dendrite structure in which an average value of arm intervals is equal to or less than 12 μm, an average grain diameter of carbides contained in the microstructure is 5 nm to 100 nm, and a number density of carbides is equal to or more than 2×10 6 /mm 2 .

Masafumi Azuma

Papers

High-strength hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having fatigue resistance, corrosion resistance, ductility and plating adhesion, after severe deformation, and a method of producing the same

Damage micromechanisms in dual-phase steel investigated with combined phase- and absorption-contrast tomography

Modelling Upper and Lower Bainite Trasformation in Steels

High-strength galvanized steel sheet, high-strength alloyed hot-dip galvanized sheet, and high-strength cold-rolled steel sheet which excel in moldability and weldability, and manufacturing method for the same

Spot-welded joint and spot welding method