Zn alloy plated steel sheet having excellent phosphatability and spot weldability and method for manufacturing same
A Zn alloy plated steel sheet having excellent phosphatability and spot weldability and a method for manufacturing the same are provided. In the Zn alloy plated steel sheet including a base steel sheet and a Zn alloy plating layer, the Zn alloy plating layer includes, by wt %, Al: 0.5-2.8%, Mg: 0.5-2.8%, and a remainder of Zn and inevitable impurities, and a cross-sectional structure of the Zn alloy plating layer includes, by area percentage, more than 50% of a Zn single phase structure and less than 50% of a Zn—Al—Mg-based intermetallic compound. A surface structure of the Zn alloy plating layer includes, by area percentage, 40% or less of a Zn single phase structure and 60% or more of a Zn—Al—Mg-based intermetallic compound.
1. A zinc (Zn) alloy plated steel sheet, the zinc alloy plated steel sheet comprising a base steel sheet and a zinc alloy plating layer,
wherein the zinc alloy plating layer includes 0.5 wt % to 2.8 wt % of aluminum (Al) and 0.5 wt % to 2.8 wt % of magnesium (Mg), with a remainder of Zn and inevitable impurities,
a cross-sectional structure of the zinc alloy plating layer includes, by area percentage, a Zn single phase structure of more than 50% (excluding 100%) and a Zn—Al—Mg-based intermetallic compound of less than 50% (excluding 0%), and
a surface structure of the zinc alloy plating layer includes, by area percentage, a Zn single phase structure of 40% or less (excluding 0%) and a Zn—Al—Mg-based intermetallic compound of 60% or more (excluding 100%).
2. The zinc alloy plated steel sheet of claim 1 , the zinc alloy plating layer includes 0.8 wt % to 2.0 wt % of Al and 0.8 wt % to 2.0 wt % of Mg, with a remainder of Zn and inevitable impurities.
3. The zinc alloy plated steel sheet of claim 1 , wherein, when an area percentage of the Zn single phase structure of the cross-sectional structure is a, and an area percentage of the Zn single phase structure of the surface structure is b, a ratio of b to a (b/a) is 0.8 or less.
4. The zinc alloy plated steel sheet of claim 1 , wherein the Zn—Al—Mg-based intermetallic compound is at least one selected from the group consisting of a Zn/Al/MgZn 2 ternary eutectic structure, a Zn/MgZn 2 binary eutectic structure, a Zn—Al binary eutectic structure, and a MgZn 2 single phase structure.
5. The zinc alloy plated steel sheet of claim 1 , wherein the Zn single phase structure includes 0.8 wt % or more of Al.
6. The zinc alloy plated steel sheet of claim 1 , wherein, when the content of Al contained in the zinc alloy plating layer is c, and the content of Al contained in the Zn single phase structure is d, a ratio of d to c (d/c) is 0.6 or more.
7. The zinc alloy plated steel sheet of claim 1 , wherein the Zn single phase structure contains 1 wt % or more of iron (Fe).
8. The zinc alloy plated steel sheet of claim 1 , wherein the sum of the contents of Al and Fe contained in the Zn single phase structure is 8 wt % or less.
9. The zinc alloy plated steel sheet of claim 1 , wherein the Zn single phase structure includes 0.1 wt % or less of Mg (including 0 wt %).
10. A method of manufacturing a zinc alloy plated steel sheet, the method comprising:
preparing a zinc alloy plating bath including 0.5 wt % to 2.8 wt % of Al and 0.5 wt % to 2.8 wt % of Mg, with a remainder of Zn and inevitable impurities;
immersing a base steel sheet in the zinc alloy plating bath, and obtaining a zinc alloy plated steel sheet by performing plating;
gas wiping the zinc alloy plated steel sheet;
primary cooling the zinc alloy plated steel sheet at a primary cooling rate of 5° C./sec or less (excluding 0° C./sec) to a primary cooling end temperature of more than 380° C. to 420° C. or less, after the gas wiping;
maintaining the zinc alloy plated steel sheet at a constant temperature for at least one second at the primary cooling end temperature, after the primary cooling; and
secondary cooling the zinc alloy plated steel sheet at a secondary cooling rate of 10° C./sec or more to a secondary cooling end temperature of 320° C. or less, after the maintaining the zinc alloy plated steel sheet at a constant temperature.
11. The method of claim 10 , further comprising: activating a surface of the base steel sheet, before the base steel sheet is immersed in the zinc alloy plating bath.
12. The method of claim 11 , wherein the activating a surface of the base steel sheet is performed by a plasma treatment or an excimer laser treatment.
13. The method of claim 11 , wherein an arithmetical average roughness Ra of the base steel sheet, having been surface activated, is 0.8 μm to 1.2 μm.
14. The method of claim 10 , wherein a temperature of the zinc alloy plating bath is from 440° C. to 460° C.
15. The method of claim 10 , wherein a surface temperature of the base steel sheet entering the zinc alloy plating bath is higher than a temperature of the zinc alloy plating bath by 5° C. to 20° C.
16. The method of claim 10 , wherein the zinc alloy plating bath includes 0.8 wt % to 2.0 wt % of Al and 0.8 wt % to 2.0 wt % of Mg, with a remainder of Zn and inevitable impurities.
17. The method of claim 10 , wherein a temperature of a wiping gas is 30° C. or more, during the gas wiping.
18. The method of claim 10 , wherein the primary cooling rate is 3° C./sec or less (excluding 0° C./sec).
19. The method of claim 10 , wherein the primary cooling end temperature is from 400° C. or more to 410° C. or less.
20. The method of claim 10 , wherein the zinc alloy plated steel sheet is maintained at the primary cooling end temperature for at least 10 seconds, during the maintaining the zinc alloy plated steel sheet at a constant temperature.
21. The method of claim 10 , wherein the secondary cooling rate is 20° C./sec or more.