Hot-rolled galvanized steel sheet having excellent galling resistance, formability and sealer-adhesion property and method for manufacturing same
Disclosed are a hot-rolled galvanized steel sheet having excellent galling resistance and formability, and a method for manufacturing the same. The hot-rolled galvanized steel sheet, includes: a base steel; and a hot-rolled galvanizing layer formed on the surface of the base steel, wherein the hot-rolled galvanizing layer provides a hot-rolled galvanized steel sheet having a Mn crystallite having a size of 10 μm or less between the resin dendrites of zinc that form sequins.
1. A method for manufacturing a hot-rolled galvanized steel sheet, the method comprising:
a plating layer forming step of depositing a steel sheet in a hot-rolled galvanizing solution, containing 0.1 to 0.8 weight percentage (wt %) of aluminum (Al), 0.05 to 1 wt % of manganese (Mn), with a remainder of zinc (Zn) and inevitable impurities, and taking out the deposited steel sheet therefrom to form a plating layer that forms a hot-rolled galvanizing layer;
a primary cooling step of cooling the steel sheet, on which the hot-rolled galvanizing layer is formed, at a cooling rate of −10 degrees Celsius per second (° C./s) until a temperature of the steel sheet reaches 420° C.;
a secondary cooling step of cooling the steel sheet at a cooling rate of −8° C./s until the temperature of the steel sheet reaches 418° C. from 420° C.; and
a tertiary cooling step of cooling the steel sheet at a steel sheet temperature of 418° C. or less at a cooling rate of −10° C./s or more to form the hot-rolled galvanizing layer.
2. The method of claim 1 , wherein the hot-rolled galvanizing solution has a temperature of 440 to 470° C.
3. The method of claim 1 , further comprising:
a wiping step of blowing nitrogen or air to the steel sheet, taken out from the hot-rolled galvanizing solution, to remove excessive molten zinc adhered to the steel sheet while cooling the steel sheet.
4. The method of claim 1 , wherein the secondary cooling step is performed by blowing a gas having a temperature ranging from 100° C. to 400° C.
5. The method of claim 4 , wherein the gas is air or a nitrogen gas.
6. The method of claim 1 , further comprising:
cleaning a surface of the steel sheet to remove foreign substances before the plating layer forming step;
annealing the steel sheet in a nitrogen-hydrogen reducing atmosphere at an A3 transformation temperature or higher; and
cooling the annealed steel sheet before being deposited in the hot-rolled galvanizing solution.
7. The method of claim 1 , further comprising:
temper-rolling a surface of a solidified hot-rolled galvanizing layer after the tertiary cooling step.
8. The method of claim 1 , wherein the hot-rolled galvanizing solution contains 0.15 to 0.5 wt % of Al, 0.05 to 0.6 wt % of Mn, and a remainder of Zn, and a total content of elements excluding Zn is 1 wt % or less.