US 6,423,426 B1 · High tensile hot-dip zinc-coated steel plate excellent in ductility and method for production thereof

Abstract

A steel sheet having a composition comprising C: 0.05-0.20 mass %, Si: 0.3-1.8 mass %, Mn: 1.0-3.0 mass %, Fe of the balance and inevitable impurities is subjected to a primary step of primary heat treatment and subsequent rapid cooling to Ms point or lower, a secondary step of secondary heat treatment and subsequent rapid cooling, and a tertiary step of galvanizing treatment and rapid cooling, so as to turn the structure of the steel sheet into a composite structure of 20% or more by volume of tempered martensite, 2% or more by volume of retained austenite, ferrite and a low-temperature transformation phase. A galvanized layer is deposited on the surface of the steel sheet. It is preferred to cool the steel sheet to 300° C. at a cooling rate of 5 ° C./sec. or more after the galvanizing treatment. After the galvanizing treatment, alloying treatment may be conducted.

Claims6

Dependent claims

A high-strength galvanized steel sheet having excellent ductility, characterized by comprising, a galvanized layer or an alloyed galvanized layer on the surface of a steel sheet,

the steel sheet having a composition comprising C:
0.05-0.20 mass %, Si:
0.3-1.8 mass %, Mn:
1.0-3.0 mass %, Fe of the balance and inevitable impurities, and comprising a composite structure having tempered martensite, retained austenite, ferrite and a low-temperature transformation phase, the tempered martensite being contained in an amount of 20% or more by volume, and the retained austenite being contained in an amount of 2% or more by volume.

1 more element

A method for manufacturing a high-strength galvanized steel sheet having excellent ductility, characterized by performing successively a primary step of subjecting steel sheet having a composition comprising C:

0.05-0.20 mass %, Si:
0.3-1.8 mass %, Mn:
1.0-3.0 mass %, Fe of the balance and inevitable impurities to primary heat treatment in which the steel sheet is held at a temperature of (Ac3 transformation temperature−50° C.) or higher for 5 seconds or more, and then cooling the steel sheet to a temperature that is not more than Ms point temperature at a cooling rate of 10° C./sec. or more;
a secondary step of subjecting the steel sheet to secondary heat treatment in which the steel sheet is held within a temperature range between Ac1 transformation temperature and Ac3 transformation temperature for 5-120 sec. and then cooling the steel sheet to a temperature that is 500° C. or lower at a cooling rate of 5° C./sec. or more;
and a subsequent tertiary step of subjecting the steel sheet to galvanized treatment to form a galvanized layer on the surface of the steel sheet and then cooling the steel sheet to 300° C. at a cooling rate of 5° C./sec. or more.

2 more elements

Dependencies are inferred from the claim text.

High tensile hot-dip zinc-coated steel plate excellent in ductility and method for production thereof

Life of this patent

High tensile hot-dip zinc-coated steel plate excellent in ductility and method for production thereof

Life of this patent

Abstract

Claims6