Steel sheet having a hot-dip Zn—Al—Mg-based coating film excellent in terms of surface appearance and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-11618938-B2
Application number	US-201816627456-A
Country	US
Kind code	B2
Filing date	Jun 6, 2018
Priority date	Jul 5, 2017
Publication date	Apr 4, 2023
Grant date	Apr 4, 2023

Abstract

A steel sheet has a hot-dip Zn—Al—Mg-based coating film, the coating film containing 1 mass % to 22 mass % of Al and 0.1 mass % to 10 mass % of Mg on a surface of the steel sheet, in which an X-ray diffraction peak intensity ratio of a Mg—Zn compound phase in the coating film, that is, MgZn 2 /Mg 2 Zn 11 , is 0.2 or less.

First claim

The invention claimed is:

1. A method of manufacturing a steel sheet having a hot-dip Zn-Al-Mg-based coating film and blackening resistance, comprising:
dipping a base steel sheet in a coating bath containing 1 mass % to 22 mass % of Al and 0.1 mass % to 10 mass % of Mg to form a hot-dip Zn-Al-Mg-based coating film,
performing primary cooling on the steel sheet coated with the hot-dip Zn-Al-Mg-based coating film to a primary cooling stop temperature of lower than 300° C. such that the coating film is completely solidified so that a MgZn 2 phase in the coating film is crystallized,
heating the cooled steel sheet to a heating temperature of 280° C. or higher and 340° C. or lower to obtain an X-ray diffraction peak intensity ratio of a Mg—Zn compound phase in the coating film, MgZn 2 /Mg 2 Zn 11 , of 0.2 or less to provide the blackening resistance, and
performing secondary cooling on the heated steel sheet,
wherein, during the heating and the secondary cooling, the steel sheet has a temperature of 250° C. or higher for time t defined by Equation (1)
18≤½×( A− 250)× t≤ 13500  (1)

where A: heating temperature (° C.) following the primary cooling and
t: time (seconds) for which the steel sheet has a temperature of 250° C. or higher in a process from the heating following the primary cooling to the secondary cooling.

2. The method according to claim 1 ,
wherein the primary cooling stop temperature is 200° C. or lower, and the heating temperature is 300° C. or higher and 340° C. or lower.

3. The method according to claim 2 , wherein the coating bath further contains 0.005 mass % to 0.25 mass % of Ni.

4. The method according to claim 3 , further comprising:
performing a chemical conversion treatment after the secondary cooling has been performed to form any one of an inorganic compound-based film, an organic resin-based film, and an inorganic compound-organic resin composite film on a surface of the coating film.

5. The method according to claim 2 , further comprising:
performing a chemical conversion treatment after the secondary cooling has been performed to form any one of an inorganic compound-based film, an organic resin-based film, and an inorganic compound-organic resin composite film on a surface of the coating film.

6. The method according to claim 1 , wherein the coating bath further contains 0.005 mass % to 0.25 mass % of Ni.

7. The method according to claim 6 , further comprising:
performing a chemical conversion treatment after the secondary cooling has been performed to form any one of an inorganic compound-based film, an organic resin-based film, and an inorganic compound-organic resin composite film on a surface of the coating film.

8. The method according to claim 1 further comprising:
performing a chemical conversion treatment after the secondary cooling has been performed to form any one of an inorganic compound-based film, an organic resin-based film, and an inorganic compound-organic resin composite film on a surface of the coating film.

Assignees

• Jfe Steel Corp

Inventors

• Harada Hiroki
• Okai Kazuhisa
• Kajiyama Hiroshi

Classifications

• C23C28/00

  Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D · CPC title

• C22C18/04

  with aluminium as the next major constituent · CPC title

• C23C22/42

  containing also phosphates · CPC title

• C23C22/78

  Pretreatment of the material to be coated · CPC title

• C23C30/00 Primary

  Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process (C23C26/00, C23C28/00 take precedence) · CPC title