1. An alloy-coated steel sheet comprising:
a steel sheet; and
an Al-Mg-Si alloy layer disposed on the steel sheet,
wherein the Al-Mg-Si alloy layer has a form in which Mg-Si alloy grains
are included in an alloy layer consisting of an Al-Mg alloy phase.
2. The alloy-coated steel sheet of claim 1, further comprising:
an Al-Si alloy layer disposed between the steel sheet and the Al-Mg-Si alloy layer.
3. The alloy-coated steel sheet of claim 1, wherein:
the Al-Mg-Si alloy layer has a gradient of a Mg content in the alloy layer.
4. The alloy-coated steel sheet of claim 1, wherein:
the Mg content in the Al-Mg-Si alloy layer is,
with respect to total 100 wt% of the Al-Mg-Si alloy layer, 15 wt% or more and 90 wt% or less.
5. The alloy-coated steel sheet of claim 1, wherein:
the Al-Mg alloy phase includes
Al3Mg2 and Al12Mg17.
6. The alloy-coated steel sheet of claim 5, wherein:
the Al-Mg alloy phase includes
Al other than Al3Mg2 and Al12Mg17, and Mg applied to the Al.
7. The alloy-coated steel sheet of claim 1, wherein:
the Mg-Si alloy grains include
Mg2Si.

8. The alloy-coated steel sheet of claim 1, wherein:
the Mg-Si alloy grains are
amorphous.
9. The alloy-coated steel sheet of claim 1, wherein:
the content of Mg-Si alloy grains in the Al-Mg-Si alloy layer is
1 wt% or more and 70 wt% or less with respect to total 100 wt% of the
Al-Mg-Si alloy layer.
10. The alloy-coated steel sheet of claim 1, further comprising:
an Al-Fe-Si alloy layer disposed between the steel sheet and the Al-Mg-Si alloy layer.
11. The alloy-coated steel sheet of claim 1, further comprising:
an Mg layer; or an Al-Mg alloy layer disposed on the Al-Mg-Si alloy layer.
12. A manufacturing method of an alloy-coated steel sheet, the
manufacturing method comprising:
preparing an aluminum plated steel sheet including a plated layer
containing Al and Si;
forming an Mg coated layer by coating Mg on the aluminum plated steel
sheet; and
diffusing Mg into the plated layer by heat-treating the aluminum plated
steel sheet coated with Mg to form an Al-Mg-Si alloy layer,
wherein the Al-Mg-Si alloy layer has a form
in which Mg-Si alloy grains are included in an alloy layer consisting of an
Al-Mg alloy phase.
13. The manufacturing method of claim 12, wherein:
in the diffusing of Mg into the plated layer by heat-treating the aluminum plated steel sheet coated with Mg,

the aluminum plated steel sheet is heat-treated at a temperature of 300°C to 450°C.
14. The manufacturing method of claim 13, wherein:
in the diffusing of Mg into the plated layer by heat-treating the aluminum
plated steel sheet coated with Mg,
the aluminum plated steel sheet is heat-treated for 5 seconds to 600
seconds.
15. The manufacturing method of claim 14, wherein:
in the diffusing of Mg into the plated layer by heat-treating the aluminum
plated steel sheet coated with Mg,
the Al-Mg alloy phase is generated in the Al-Mg-Si alloy layer, and
the Al-Mg alloy phase includes
Al3Mg2 and Al12Mg17.
16. The manufacturing method of claim 14, wherein:
in the diffusing of Mg into the plated layer by heat-treating the aluminum
plated steel sheet coated with Mg,
the Mg-Si alloy grains are generated in the Al-Mg-Si alloy layer, and
the Mg-Si alloy grains include
Mg2Si.
17. The manufacturing method of claim 12, wherein:
the forming of the Mg coated layer by coating Mg on the aluminum
plated steel sheet,
is performed by physical vapor deposition (PVD).