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

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.