High-strength Zn—Al coated steel wire for bridges with excellent corrosion resistance and fatigue properties and method for manufacturing the same

1. A high-strength Zn—Al coated steel wire for bridges comprising:

a steel wire; and

a Zn—Al coating having a coating body layer and an Fe—Al alloy layer formed in an interface between a surface layer of the steel wire and the coating body layer,

wherein a chemical composition of a core material of the steel wire consists of, by mass %:

C: 0.70% to 1.2%;

Si: 0.01% to 2.5%;

Mn: 0.01% to 0.9%;

P: limited to 0.02% or less;

S: limited to 0.02% or less;

N: limited to 0.01% or less; and

a balance including Fe and unavoidable impurities,

wherein wire-drawn pearlite is the most abundant microstructure among microstructures of the core material of the steel wire;

wherein an average composition of the Zn—Al coating consists of, by mass %:

Al: 3.0 to 15.0%;

Si: 0.2% to 1.5%;

Fe: limited to 3.0% or less, and

a balance including Zn and unavoidable impurities,

wherein the Fe—Al alloy layer has a thickness of 5 μm or less and comprises an outer layer and an inner layer,

wherein an Al 3.2 Fe columnar grain is the most abundant microstructure among microstructures in the outer layer,

wherein Al 5 Fe 2 columnar grain is the most abundant microstructure among microstructures in the inner layer,

wherein the outer layer is located closer to the coating body layer than the inner layer, and the inner layer is located closer to the surface layer of the steel wire than the outer layer,

wherein the Zn—Al coating further comprises an Fe—Al—Si granular grain layer between the outer layer of the Fe—Al alloy layer and the coating body layer.

2. The high-strength Zn—Al coated steel wire for bridges according to claim 1 ,

wherein a primary phase of the coating body layer has a diameter of 10 μm or less, and

wherein the wire-drawn pearlite comprises 90% or more of the microstructures of the core material of the steel wire.

3. The high-strength Zn—Al coated steel wire for bridges according to claim 1 or 2 , wherein the average composition of the Zn—Al coating includes, by mass %,

Al: 6.0 to 15.0%.

4. The high-strength Zn—Al coated steel wire for bridges according to claim 1 or 2 , wherein a torsion number before fracture during a torsion test is at least 18 times.

5. The high-strength Zn—Al coated steel wire for bridges according to claim 1 or 2 , wherein a ratio of fatigue limit under partial pulsating tension to tensile strength is 0.22 or higher.

6. A method for manufacturing a high-strength Zn—Al coated steel wire for bridges according to claim 1 or 2 , the method comprising:

wire-drawing the steel wire at a temperature of 250° C. or less;

pickling the steel wire with acid;

treating the steel wire with a flux; and

coating the flux-treated steel wire with Zn—Al,

wherein the coating with Zn—Al is the only coating process comprised in the method for manufacturing the coated steel wire.

7. The method for manufacturing the high-strength Zn—Al coated steel wire for bridges according to claim 6 , wherein the coating with Zn—Al is carried out by immersing the wire-drawn steel wire in molten Zn—Al bath containing, by mass %, Al: 3.0% to 15.0%.

8. The method for manufacturing the high-strength Zn—Al coated steel wire for bridges according to claim 6 , the method further comprising:

hot-rolling the steel material; and

patenting the steel material by immersion in salt bath at 500° C. to 600° C. after the hot-rolling.

9. The method for manufacturing the high-strength Zn—Al coated steel wire for bridges according to claim 6 , wherein the coating with Zn—Al is carried out by immersing the wire-drawn steel wire in a molten Zn—Al bath, wherein

the molten Zn—Al bath further contains, by mass %,

Si: 1.5% or less, and

the average composition of Zn—Al coating further comprises, by mass %,

Si: 0.01% to 1.5%.

10. The method for manufacturing the high-strength Zn—Al coated steel wire for bridges according to claim 7 , wherein

the molten Zn—Al bath contains, by mass %,

Al: 6.0% to 15%, and

the average composition of Zn—Al coating includes, by mass %,

Al: 6.0 to 15.0%.

11. The method for manufacturing the high-strength Zn—Al coated steel wire for bridges according to claim 6 , the method further comprising, after the wire-drawing, at least one of:

roller-straightening; and

heating the steel wire at 400° C. to 500° C. for 1 sec to 60 sec.

12. A high-strength Zn—Al coated steel wire for bridges comprising:

a steel wire; and

a Zn—Al coating having a coating body layer and an Fe—Al alloy layer formed in an interface between a surface layer of the steel wire and the coating body layer,

wherein a chemical composition of a core material of the steel wire consists of, by mass %:

C: 0.70% to 1.2%;

Si: 0.01% to 2.5%;

Mn: 0.01% to 0.9%;

P: limited to 0.02% or less;

S: limited to 0.02% or less;

N: limited to 0.01% or less;

Cr: 0% to 0.5%,

Ni: 0% to 1.0%,

Cu: 0% to 0.5%,

Mo: 0% to 0.5%,

V: 0% to 0.5%,

B: 0% to 0.0070%,

Al: 0% to 0.1%,

Ti: 0% to 0.1%,

Nb: 0% to 0.05%,

Zr: 0% to 0.1%, and

a balance including Fe and unavoidable impurities,

wherein wire-drawn pearlite is the most abundant microstructure among microstructures of the core material of the steel wire,

wherein an average composition of the Zn—Al coating consists of, by mass %:

Al: 3.0 to 15.0%;

Si: 0.2% to 1.5%;

Fe: limited to 3.0% or less, and

a balance including Zn and unavoidable impurities,

wherein the Fe—Al alloy layer has a thickness of 5 μm or less and comprises an outer layer and an inner a layer,

wherein an Al 3.2 Fe columnar grain is the most abundant microstructure among microstructures in the outer layer,

wherein Al 5 Fe 2 columnar grain is the most abundant microstructure among microstructures in the inner layer,

wherein the outer layer is located closer to the coating body layer than the inner layer, and the inner layer is located closer to the surface layer of the steel wire than the outer layer,

wherein the Zn—Al coating further comprises an Fe—Al—Si granular grain layer between the outer layer of the Fe—Al alloy layer and the coating body layer.