High-strength galvanized steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12529131-B2
Application number	US-202017791322-A
Country	US
Kind code	B2
Filing date	Jan 10, 2020
Priority date	Jan 10, 2020
Publication date	Jan 20, 2026
Grant date	Jan 20, 2026

Abstract

A high-strength galvanized steel sheet is disclosed which has a specified chemical composition, has a steel microstructure including, in terms of area fraction, 35% or more and 80% or less of ferrite, 0.1% or more and less than 5.0% of as-quenched martensite, 3.0% or more and 35% or less of tempered martensite, and 8% or more of retained austenite, in which an average grain diameter of the ferrite is 6 μm or less, in which an average grain diameter of the retained austenite is 3 μm or less, in which a value calculated by dividing an average Mn content (mass %) in the retained austenite by an average Mn content (mass %) in the ferrite is 1.5 or more.

First claim

The invention claimed is:

1 . A high-strength galvanized steel sheet having a chemical composition containing, by mass %,
C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 3.10% to 8.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005% to 0.0100%, Al: 0.001% to 2.000%, and a balance of Fe and incidental impurities, a steel microstructure including, in terms of area fraction, 35% or more and 80% or less of ferrite, 0.1% or more and less than 5.0% of as-quenched martensite, 3.0% or more and 35% or less of tempered martensite, and 8% or more of retained austenite, in which an average grain diameter of the ferrite is 6 μm or less, in which an average grain diameter of the retained austenite is 3 μm or less, in which a value calculated by dividing an average Mn content (mass %) in the retained austenite by an average Mn content (mass %) in the ferrite is 1.5 or more, in which a value calculated by dividing a sum of an area fraction of tempered martensite having a circle-equivalent grain diameter of 3 μm or less and an area fraction of retained austenite having a circle-equivalent grain diameter of 3 μm or less by a sum of an area fraction of all the tempered martensite and an area fraction of all the retained austenite is 0.4 or more, and in which a value calculated by dividing an area fraction of retained austenite grains adjacent to three or more ferrite grains having different crystal orientations by the area fraction of all the retained austenite is less than 0.6, a galvanizing layer on a surface of the steel sheet, and a diffusible hydrogen content in steel of 0.3 mass ppm or less.

2 . The high-strength galvanized steel sheet according to claim 1 , wherein the chemical composition further contains, by mass %, at least one selected from
Ti: 0.005% to 0.200%, Nb: 0.005% to 0.200%, V: 0.005% to 0.500%, W: 0.005% to 0.500%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.100%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0050%.

3 . The high-strength galvanized steel sheet according to claim 1 , wherein the galvanizing layer is a galvannealing layer.

4 . The high-strength galvanized steel sheet according to claim 2 , wherein the galvanizing layer is a galvannealing layer.

5 . A method for manufacturing the high-strength galvanized steel sheet according to claim 1 , the method comprising heating a steel slab having the chemical composition, performing hot rolling on the heated slab with a finishing delivery temperature of 750° C. or higher and 1000° C. or lower, coiling the hot-rolled steel sheet at a temperature of 300° C. or higher and 750° C. or lower, performing cold rolling on the hot-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range equal to or higher than an Aca transformation temperature for 20 s to 1800 s, cooling the held steel sheet to a cooling stop temperature of 50° C. or higher and 350° C. or lower, holding the cooled steel sheet at the cooling stop temperature for 2 s to 600 s, cooling the held steel sheet, holding the cooled steel sheet in a temperature range equal to or higher than an Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, performing a galvanizing treatment, holding the galvanized steel sheet in a temperature range of 50° C. or higher and 400° C. or lower for 1800 s or more, and cooling the held steel sheet.

6 . A method for manufacturing the high-strength galvanized steel sheet according to claim 2 , the method comprising heating a steel slab having the chemical composition, performing hot rolling on the heated slab with a finishing delivery temperature of 750° C. or higher and 1000° C. or lower, coiling the hot-rolled steel sheet at a temperature of 300° C. or higher and 750° C. or lower, performing cold rolling on the hot-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range equal to or higher than an Aca transformation temperature for 20 s to 1800 s, cooling the held steel sheet to a cooling stop temperature of 50° C. or higher and 350° C. or lower, holding the cooled steel sheet at the cooling stop temperature for 2 s to 600 s, cooling the held steel sheet, holding the cooled steel sheet in a temperature range equal to or higher than an Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, performing a galvanizing treatment, holding the galvanized steel sheet in a temperature range of 50° C. or higher and 400° C. or lower for 1800 s or more, and cooling the held steel sheet.

7 . A method for manufacturing the high-strength galvanized steel sheet according to claim 1 , the method comprising heating a steel slab having the chemical composition, performing hot rolling on the heated slab with a finishing delivery temperature of 750° C. or higher and 1000° C. or lower, coiling the hot-rolled steel sheet at a temperature of 300° C. or higher and 750° C. or lower, performing cold rolling on the hot-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range equal to or higher than an Aca transformation temperature for 20 s to 1800 s, cooling the held steel sheet to a cooling stop temperature of 50° C. or higher and 350° C. or lower, holding the cooled steel sheet at the cooling stop temperature for 2 s to 600 s, cooling the held steel sheet, holding the cooled steel sheet in a temperature range equal to or higher than an Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, again holding the cooled steel sheet in a temperature range equal to or higher than the Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, performing a galvanizing treatment, holding the galvanized steel sheet in a temperature range of 50° C. or higher and 400° C. or lower for 1800 s or more, and cooling the held steel sheet.

8 . A method for manufacturing the high-strength galvanized steel sheet according to claim 2 , the method comprising heating a steel slab having the chemical composition, performing hot rolling on the heated slab with a finishing delivery temperature of 750° C. or higher and 1000° C. or lower, coiling the hot-rolled steel sheet at a temperature of 300° C. or higher and 750° C. or lower, performing cold rolling on the hot-rolled steel sheet, holding the cold-rolled steel sheet in a temperature range equal to or higher than an Aca transformation temperature for 20 s to 1800 s, cooling the held steel sheet to a cooling stop temperature of 50° C. or higher and 350° C. or lower, holding the cooled steel sheet at the cooling stop temperature for 2 s to 600 s, cooling the held steel sheet, holding the cooled steel sheet in a temperature range equal to or higher than an Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, again holding the cooled steel sheet in a temperature range equal to or higher than the Ac 1 transformation temperature and equal to or lower than (Ac 1 transformation temperature+150° C.) for 20 s to 1800 s, cooling the held steel sheet, performing a galvanizing treatment, holding the galvanized steel sheet in a temperature range of 50° C. or higher and 400° C. or lower for 1800 s or more, and cooling the held steel sheet.

9 . The method

Assignees

• Jfe Steel Corp

Inventors

• ENDOH Kazuki
• Kawasaki Yoshiyasu
• Toji Yuki
• Funakawa Yoshimasa
• Aoyama Mai

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title

• C22C38/001

  containing N · CPC title