Hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet having improved hole expansion ratio, and manufacturing methods thereof

• US 10,351,924 B2
• Filed: 05/08/2015
• Issued: 07/16/2019
• Est. Priority Date: 12/19/2014
• Status: Active Grant

• Current Assignee: POSCO Co. Ltd. (Posco Holdings Incorporated)
• Original Assignee: Posco Holdings Incorporated
• Inventors: Han, Sang-Ho, Han, Seong-Ho
• Primary Examiner(s): Wu, Jenny R
• Application Number:
  US14/888,394
• Publication Number:
  US 20160340755A1
• Time in Patent Office: 1,530 Days
• CPC Class Codes: B32B 15/01 all layers being exclusively metallic making layered metal workpieces by pressure cladding B23K20/22; making coatings with a metallic material characterised by its composition C23C30/00
• CPC Class Codes: B32B 15/013 one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• CPC Class Codes: C21D 1/74 Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 2251/02 Clad material
• CPC Class Codes: C21D 8/0221 characterised by the working steps
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0236 Cold rolling
• CPC Class Codes: C21D 8/0247 characterised by the heat treatment
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C21D 8/0278 involving a particular surface treatment C21D8/0294 takes precedence
• CPC Class Codes: C21D 9/46 for sheet metals
• CPC Class Codes: C21D 9/561 with a controlled atmosphere or vacuum
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/002 containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/22 : with molybdenum or tungsten
• CPC Class Codes: C22C 38/32 : with boron
• CPC Class Codes: C22C 38/38 : with more than 1.5% by weight of manganese
• CPC Class Codes: C23C 2/02 : Pretreatment of the material to be coated, e.g. for coating on selected surface areas C23C2/30 takes precedence
• CPC Class Codes: C23C 2/06 : Zinc or cadmium or alloys based thereon
• CPC Class Codes: C23C 2/285 : for remelting the coating
• CPC Class Codes: C23C 2/29 : Cooling or quenching

Abstract

A hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on the base steel sheet, the base steel sheet including, by wt %, C:0.02% to 0.08%, Mn:1.3% to 2.1%, Si:0.3% or less (excluding 0%), Cr:1.0% or less (excluding 0%), P:0.1% or less (excluding 0%), S: 0.01% or less (excluding 0%), N:0.01% or less (excluding 0%), sol.Al: 0.02% to 0.06%, Mo:0.2% or less (excluding 0%), B:0.003% or less (excluding 0%), and a balance of Fe and inevitable impurities, wherein the base steel sheet has a microstructure comprising 90% or more by area of ferrite, 3% or less by area of bainite as described in Formula 1, and martensite as a remainder.

Claims

1. A hot-dip galvanized steel sheet having an improved hole expansion ratio, the hot-dip galvanized steel sheet comprising

a base steel sheet and

an exposed hot-dip galvanized surface layer formed on and covering the base steel sheet,

the base steel sheet comprising, by wt %,

carbon (C) having an amount [C] satisfying 0.02%≤

[C]≤

0.08%,

manganese (Mn) having an amount [Mn] satisfying 1.3%≤

[Mn]≤

2.1%,

silicon (Si) having an amount [Si] satisfying 0<

[Si]≤

0.3%,

chromium (Cr) having an amount [Cr] satisfying 0<

[Cr]≤

1.0%,

phosphorus (P) having an amount [P] satisfying 0<

[P]≤

0.1%,

sulfur (S) having an amount [S] satisfying 0<

[S]≤

0.01%,

nitrogen (N) having an amount [N] satisfying 0<

[N]≤

0.01%,

acid soluble aluminum (sol. Al) having an amount [sol. Al] satisfying 0.02%≤

[sol. Al]≤

0.06%,

molybdenum (Mo) having an amount [Mo] satisfying 0<

[Mo]≤

0.2%,

boron (B) having an amount [B] satisfying 0<

[B]≤

0.003%, and

a balance of iron (Fe) and inevitable impurities,

wherein the base steel sheet has a microstructure comprising

more than 90% by area of ferrite,

3% or less by area of bainite as described in Formula 1 below, and

martensite as a remainder,

wherein the base steel sheet has an a/b ratio of 0.7 or less, where (a) refers to a C+Mn content of a matrix of the base steel sheet in a surface region defined up to an average depth of 10 μ

m, and (b) refers to a C+Mn content of the matrix at a ¼

thickness position of the base steel sheet,

Bainite (%)={BA/(FA+MA+BA)}×

100 

 

[Formula 1]

where FA;

ferrite area,

BA;

bainite area, and

MA;

martensite area.

2. The hot-dip galvanized steel sheet of claim 1, wherein the base steel sheet has a c/d ratio of 0.8 or less, where (c) refers to a martensite area fraction of the matrix of the base steel sheet in the surface region defined up to an average depth of 10 μ

m, and (d) refers to a martensite area fraction of the matrix of the base steel sheet at the ¼

thickness position of the base steel sheet.

3. The hot-dip galvanized steel sheet of claim 1, wherein the base steel sheet has an e/f ratio of 0.95 or less, where (e) refers to a C+Mn content of martensite in the surface region of the base steel sheet defined up to an average depth of 10 μ

m, and (f) refers to a C+Mn content of martensite at the ¼

thickness position of the base steel sheet.

4. The hot-dip galvanized steel sheet of claim 1, wherein the base steel sheet has a g/h ratio of 1.2 or greater, where (g) refers to a ferrite grain size in the surface region of the base steel sheet defined up to an average depth of 10 μ

m, and (h) refers to a ferrite grain size at the ¼

thickness position of the base steel sheet.

5. The hot-dip galvanized steel sheet of claim 1, wherein the hot-dip galvanized surface layer of the hot-dip galvanized steel sheet is subjected to an alloying treatment process to form the hot-dip galvanized steel sheet as a hot-dip galvannealed steel sheet.

6. A method of manufacturing a hot-dip galvanized steel sheet having an improved hole expansion ratio HER, the method comprising:

preparing a steel slab and reheating the steel slab, the steel slab comprising, by wt %,

carbon (C) having an amount [C] satisfying 0.02%≤

[C]≤

0.08%,

manganese (Mn) having an amount [Mn] satisfying 1.3%≤

[Mn]≤

2.1%,

silicon (Si) having an amount [Si] satisfying 0<

[Si]≤

0.3%,

chromium (Cr) having an amount [Cr] satisfying 0<

[Cr]≤

1.0%,

phosphorus (P) having an amount [P] satisfying 0<

[P]≤

0.1%,

sulfur (S) having an amount [S] satisfying 0<

[S]≤

0.01%,

nitrogen (N) having an amount [N] satisfying 0<

[N]≤

0.01%,

acid soluble aluminum (sol. Al) having an amount [sol. Al] satisfying 0.02%≤

[sol. Al]≤

0.06%,

molybdenum (Mo) having an amount [Mo] satisfying 0<

[Mo]≤

0.2%,

boron (B) having an amount [B] satisfying 0<

[B]≤

0.003%, and

a balance of iron (Fe) and inevitable impurities,

performing a finish hot rolling process on the reheated steel slab within a temperature range of Ar3+50°

C. to 950°

C. to form a hot-rolled steel sheet, and coiling the hot-rolled steel sheet within a temperature range of 450°

C. to 700°

C.;

cold rolling the coiled steel sheet with a reduction ratio of 40% to 80%, and continuously annealing the cold-rolled steel sheet in a furnace while maintaining a partial pressure of hydrogen in the furnace as expressed by −

4.0≤

log(PH2O/PH2)≤

−

2.0, wherein in the continuous annealing, the cold-rolled steel sheet is first heated to a temperature of 560°

C. to 680°

C. at an average heating rate of 4°

C./s or greater and is secondarily heated to a temperature of 760°

C. to 850°

C. at an average heating rate of 2°

C./s or less;

first cooling the continuously annealed steel sheet to a temperature range of 630°

C. to 670°

C. at an average cooling rate of 2°

C./s to 8°

C./s and secondarily cooling the steel sheet to a temperature ranging from Ms+20°

C. to Ms+50°

C. at an average cooling rate of 3°

C./s to 10°

C./s; and

performing a hot-dip galvanizing process on the secondarily cooled steel sheet within a temperature range of 440°

C. to 480°

C., and cooling the hot-dip galvanized steel sheet to a temperature of Ms−

100°

C. or lower at an average cooling rate of 4°

C./s or greater,

wherein Ms refers to a theoretical temperature at which martensite (M) is formed, and

wherein the hot-dip galvanized steel sheet includes an exposed surface layer and a covered base steel sheet, and the base steel sheet has a microstructure comprising;

more than 90% by area of ferrite,

3% or less by area of bainite as described in Formula 1 below, and

martensite as a remainder, and

the base steel sheet has an a/b ratio of 0.7 or less, where (a) refers to a C+Mn content of a matrix of the base steel sheet in a surface region defined up to an average depth of 10 μ

m, and (b) refers to a C+Mn content of the matrix at a ¼

thickness position of the base steel sheet,

Bainite (%)={BA/(FA+MA+BA)}×

100 

 

[Formula 1]

where FA;

ferrite area,

BA;

bainite area, and

MA;

martensite area.

7. The method of claim 6, wherein the base steel sheet of the hot-dip galvanized steel sheet has a c/d ratio of 0.8 or less, where (c) refers to a martensite area fraction of the matrix of the base steel sheet in the surface region defined up to an average depth of 10 μ

m, and (d) refers to a martensite area fraction of the matrix at the ¼

thickness position of the base steel sheet.

8. The method of claim 6, wherein the base steel sheet of the hot-dip galvanized steel sheet has an e/f ratio of 0.95 or less, where (e) refers to a C+Mn content of martensite in the surface region of the base steel sheet defined up to an average depth of 10 μ

m, and (f) refers to a C+Mn content of martensite at the ¼

thickness position of the base steel sheet.

9. The method of claim 6, wherein the base steel sheet of the hot-dip galvanized steel sheet has a g/h ratio of 1.2 or greater, where (g) refers to a ferrite grain size in the surface region of the base steel sheet defined up to an average depth of 10 μ

m, and (h) refers to a ferrite grain size at the ¼

thickness position of the base steel sheet.

10. The method of claim 6, wherein the partial pressure of hydrogen satisfies −

3.0≤

log(PH2O/PH2)≤

−

2.5.

11. The method of claim 6, further comprising performing an alloying heat treatment process after the hot-dip galvanizing process.