Hot-dip Sn-Zn coated steel sheet having excellent corrosion resistance

1. A method for manufacturing a hot-dip Sn—Zn coated steel sheet, comprising:

pre-coating a Fe—Ni alloy containing between about 10 to 80 mass % of Ni, and a remainder being Fe on a steel sheet;

immersing the steel sheet into a Sn—Zn hot-dip coating bath so as to form a hot-dip coating layer on the steel sheet;

cooling the hot-dip coating layer at a cooling rate of about 10° C./sec to about 30° C./sec so as to obtain a hot-dip Sn—Zn coated steel sheet;

measuring a differential scanning calorimetric curve of the hot-dip Sn—Zn coated steel sheet by a differential scanning calorimetry; and

distinguishing whether the hot-dip Sn—Zn coated steel sheet has an objective microstructure by the measured differential scanning calorimetric curve,

wherein the objective microstructure satisfies the following formula:

(endothermic value of melting heat generated by Sn primary crystals)/{(endothermic value of melting heat generated by Sn primary crystals)+(endothermic value of melting heat generated by Sn—Zn eutectic crystals)}≧0.3.

2. The method according to claim 1 , wherein the steel sheet is immersed into the Sn—Zn hot-dip coating bath under conditions where the hot-dip coating bath temperature is in a range between about 240° C. and 300° C. and the immersion time is in a range of about 2 to 15 seconds so as to form the hot-dip coating layer comprising between about 1 mass % and 8.8 mass % of Zn and a remainder comprising between about 91.2 mass % and 99.0 mass % of Sn and inevitable impurities on a surface of the steel sheet.

3. The method according to claim 1 , wherein the objective microstructure has properties such that a temperature of an endothermic peak generated by the Sn primary crystals melting is between about 200° C. and 230° C., and a temperature of an endothermic peak generated by the Sn—Zn eutectic crystals melting is between about 198° C. and 200° C.

4. The method according to claim 1 , wherein the hot-dip Sn—Zn coated steel sheet including the hot-dip coating layer having the objective microstructure has an outer surface corrosion resistance exhibiting about 10% or less of an area ratio of red rust occurring after a salt spray test, an SST of 960 hours and has an inner surface corrosion resistance exhibiting less than about 200 ppm of an elution amount of metal ions in a corrosion test.

5. The method according to claim 1 , further comprising conducting a corrosion test by:

leaving a gasoline at about 100° C. for approximately 24 hours in a pressure vessel to prepare a forcedly-deteriorated gasoline;

adding about 10 vol. % of water to the forcedly-deteriorated gasoline to prepare a corrosive liquid;

forming a sample of a coated steel sheet having an approximately 30×35 mm of edge and rear face seals with a draw-bead at a reduction rate of sheet thickness of about 14%;

corroding the sample in 350 ml of the corrosive liquid at about 45° C. for about three weeks; and

measuring an elution amount of eluted metal ions.

6. The method according to claim 1 , wherein the hot-dip coating layer comprises between about 4 mass % and 8.8 mass % of Zn and a remainder comprises between about 91.2 mass % and 96.0 mass % of Sn and inevitable impurities.