Hot-dipped steel and method of producing same

1. A hot-dipped steel comprising a steel substrate with an aluminum-zinc alloy plating layer formed thereon,

said aluminum-zinc alloy plating layer containing Al, Zn, Si and Mg as constituent elements thereof,

wherein

said aluminum-zinc alloy plating layer contains 0.1% to 10% by weight of Mg,

said aluminum-zinc alloy plating layer contains 0.2% to 15% by volume of an Si—Mg phase,

the weight ratio of Mg in the Si—Mg phase to the total weight of Mg is 3% or more, and

the aluminum-zinc alloy plating layer further contains 0.02% by weight to 1.0% by weight of Cr as a constituent element thereof,

protrusions having height of greater than 200 μm and steepness greater than 1.0 are no longer present on a surface of the aluminum-zinc alloy plating layer.

2. The hot-dipped steel according to claim 1 , wherein said aluminum-zinc alloy plating layer contains 25% to 75% by weight of Al, and 0.5% to 10% by weight of Si with respect to the weight of Al; and

the weight ratio of Si to Mg is between 100:50 and 100:300.

3. The hot-dipped steel according to claim 1 , wherein the aluminum-zinc alloy plating layer further contains at least one of Ti and B, and total amount of Ti and B is within a range of 0.0005% to 0.1% by weight.

4. The hot-dipped steel according to claim 1 , wherein the aluminum-zinc alloy plating layer further contains 1 ppm to 1000 ppm by weight of Sr.

5. The hot-dipped steel according to claim 1 , wherein the Mg content in any region having a size of 4 mm in diameter and a depth of 50 nm in an outermost layer of the aluminum-zinc alloy plating layer having a depth of 50 nm is less than 60% by weight.

6. The hot-dipped steel according to claim 1 , wherein the content of Cr in an outermost layer of the aluminum-zinc alloy plating layer having a depth of 50 nm is within a range of 100 ppm by weight to 500 ppm by weight.

7. The hot-dipped steel according to claim 1 , wherein an alloy layer containing Al and Cr is interposed between the aluminum-zinc alloy plating layer and the steel substrate, and the ratio of the weight proportion of Cr in the alloy layer to the weight ratio of Cr in the aluminum-zinc alloy plating layer is within a range of 2 to 50.

8. The hot-dipped steel according to claim 1 , wherein

said aluminum-zinc alloy plating layer contains said Si—Mg phase in the surface thereof at a surface area ratio of 30% or less.

9. A method of producing the hot-dipped steel according to claim 1 , said method comprising:

preparing a hot-dip plating bath containing an alloy composition containing,

25% to 75% by weight of Al,

0.1% to 10% by weight of Mg,

0.02% to 1.0% by weight of Cr,

0.5% to 10% by weight, based on Al, of Si,

1 ppm to 1000 ppm by weight of Sr,

0.1% to 1.0% by weight of Fe,

the remainder being Zn,

the weight ratio of Si to Mg being 100:50 to 100:300;

passing a steel substrate through said hot-dip plating bath to deposit a hot-dip plating metal on a surface thereof; and

solidifying said hot-dip plating metal to form an aluminum-zinc alloy plating layer on the surface of the steel substrate,

wherein said aluminum-zinc alloy plating layer contains 0.2% to 15% volume of an Si—Mg phase, wherein the weight ratio of Mg in the Si—Mg phase to the total weight of Mg is 3% or more, and wherein protrusions having height of greater than 200 μm and steepness greater than 1.0 are no longer present on a surface of the aluminum-zinc alloy plating layer.

10. The method according to claim 9 , wherein the hot-dip plating bath further contains 100 ppm to 5000 ppm by weight of Ca.

11. The method according to claim 9 , wherein the hot-dip plating bath further contains at least one of Ti and B within a range 0.0005% to 0.1% by weight.

12. The method according to claim 9 , wherein said hot-dip plating bath is maintained, at a temperature not exceeding by 40° C. above a solidification starting temperature of said alloy composition.

13. The method according to claim 9 , wherein said steel substrate is transferred from said hot-dip plating bath to a non-oxidative atmosphere or low oxidative atmosphere, after which a gas wiping process is made to adjust an amount of the hot-dip plating metal deposited on said steel substrate in said non-oxidative atmosphere or low oxidative atmosphere before said hot-dip plating metal is solidified.

14. The method according to claim 9 , further including a step of holding said steel substrate coated with the aluminum-zinc alloy plating layer, at a holding temperature t (° C.) for a holding time y (hr) defined by the following formula (I):

5.0×10 22 ×t −10.0 ≦y≦ 7.0×10 24 ×t −10.0   (I)

(where 150≦t≦250).