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

Patent metadata

Field	Value
Publication number	US-10351924-B2
Application number	US-201514888394-A
Country	US
Kind code	B2
Filing date	May 8, 2015
Priority date	Dec 19, 2014
Publication date	Jul 16, 2019
Grant date	Jul 16, 2019

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.

First claim

The invention claimed is:

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 μm, 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 μm, 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 μm, 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 μm, 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 μm, 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 μm, 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 μm, 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 μm, 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.

Assignees

• Posco

Inventors

• Han Sang-Ho
• Han Seong-Ho

Classifications

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C22C38/22

  with molybdenum or tungsten · CPC title

• C21D9/561

  with a controlled atmosphere or vacuum · CPC title

• C21D8/0247

  characterised by the heat treatment · CPC title