JP2009024210A - Hot-dip zinc alloy plated steel wire - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip zinc alloy plated steel wire having both of superior corrosion resistance and workability. <P>SOLUTION: The hot-dip zinc alloy plated steel wire has a hot-dip plated layer thereon which has an average composition comprising, by mass%, 4 to 20% of Al, 0.1 to 1.0% of Mg and the balance Zn with unavoidable impurities. The hot-dip plated layer includes an α phase which has an aspect ratio L/C of a length (L) in an axial direction of the steel wire to a length (C) in a cross direction in an amount of 2.0 or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a hot dip galvanized steel wire with improved workability and corrosion resistance, which is used by being exposed to the outdoors, especially for aquaculture facilities and structures exposed to seawater and the like in a highly corrosive environment.

In general, hot dip galvanizing is performed as a method for imparting corrosion resistance to steel wires. However, ZnAl alloy plated steel wires plated with hot dip ZnAl alloy with further improved corrosion resistance are used in various corrosive environments. However, even a ZnAl alloy-plated steel wire has a limited corrosion resistance and has a short life when used in a wet highly corrosive environment immersed in seawater or the like. For this reason, a molten ZnAlMg alloy-plated steel wire containing magnesium is used as a car mat for river revetment work, etc., as it is more excellent in corrosion resistance than a molten ZnAl alloy-plated steel wire. Thus, the hot-dip ZnAlMg alloy plating steel wire which further improved corrosion resistance is proposed, for example in patent documents 1-3.
Japanese Patent No. 3854468 Japanese Patent No. 3857882 Japanese Patent No. 3599680

  However, the hot-dip ZnAlMg alloy plating of Patent Documents 1 to 3 has a defect that the workability is inferior because the Fe—Zn alloy layer is harder than hot-dip zinc plating or hot-dip ZnAl alloy plating. That is, when wire drawing, rolling, or bending is performed after plating, fine cracks are generated in the alloy layer, which grows and becomes cracks or delamination, thereby impairing the function of the anticorrosion coating layer.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a hot dip galvanized steel wire having both excellent corrosion resistance and workability.

  The hot dip galvanized steel wire according to the present invention has an average composition of mass%, Al: 4 to 20%, Mg: 0.1 to 1.0%, and the remainder comprising Zn and inevitable impurities. And an α phase in which the aspect ratio L / C between the length L in the axial direction (drawing direction) of the steel wire and the length C in the cross direction is 2.0 or more is included in the hot-dip plated layer It is characterized by that.

  By making the aspect ratio L / C of the α phase in the hot-dipped layer 2.0 or more, workability is improved, and cracks and peeling do not occur in the plated layer. In this way, the elongated α phase is formed by performing wire drawing, cold rolling, or skin pass (temper rolling) as a surface reduction with a large total area reduction. As the true strain ε increases, the aspect ratio L / C of the α phase also increases. For example, when surface reduction processing is performed with the true strain ε of 0.40 or more, the aspect ratio L / C of the α phase is approximately 2 0 or more (FIG. 4).

  Furthermore, in the above-described plated steel wire, it is preferable that the average thickness of the Fe—Zn alloy layer formed between the hot-dip plated layer and the steel wire is 10 μm or less. In order to adjust the average thickness of the Fe—Zn alloy layer to 10 μm or less, the immersion time of the steel wire in the plating bath, the temperature of the plating bath, and the composition of the plating bath are controlled in desired ranges, respectively. In particular, the thickness of the Fe—Zn alloy layer is strongly dependent on the immersion time of the steel wire in the plating bath. Therefore, the thickness of the Fe—Zn alloy layer can be adjusted to a desired value by controlling the immersion time with high accuracy. When the average thickness of the Fe—Zn alloy layer is 10 μm or less, the workability is greatly improved (FIG. 8).

  According to the present invention, by controlling the alpha phase aspect ratio (L / C) in the plating layer to an elongated shape of 2.0 or more, a winding test of a self-diameter wire that has cracked or peeled off in the conventional product. Thus, the plating surface after the winding test can obtain excellent workability equivalent to that shown in FIG. In the present invention, even if a crack occurs in the alloy layer, since the progress of the crack is prevented by the elongated α phase having an aspect ratio of 2.0 or more in the plating layer, the crack propagates in the plating layer. Absent.

  The hot dip galvanized steel wire of the present invention comprises a hot dip plated layer having an average composition of mass%, Al: 4 to 20%, Mg: 0.1 to 1.0%, the balance being Zn and inevitable impurities. In this hot-dip plated layer, there is an α phase having an aspect ratio L / C of 2.0 or more between the length L in the longitudinal axis direction (drawing direction) of the steel wire and the length C in the cross direction. It is included.

  Hereinafter, the reason for limitation of each component of the present invention will be described.

1) Al: 4 to 20% by mass
Al has the effect of increasing the corrosion resistance. Further, Al has an effect of preventing oxidation of other elements in the plating layer. When the amount of Al is less than 4% by mass, the effect of preventing oxidation of Mg in the plating bath cannot be obtained. On the other hand, if the Al content exceeds 20% by mass, the plating layer becomes hard and brittle, and the workability deteriorates. Therefore, in the present invention, the Al amount is set in the range of 4 mass% or more and 20 mass% or less.

2) Mg: 0.1 to 1.0% by mass
Mg is an important element having an effect of improving corrosion resistance that promotes the growth of uniform corrosion products and prevents the progress of local corrosion. When the Mg content is less than 0.1% by mass, the corrosion resistance decreases. Further, if the amount of Mg is insufficient, the corrosion resistance is remarkably lowered, and for example, the basic corrosion resistance level required in a revetment car mat facility cannot be satisfied.

  On the other hand, when the amount of Mg exceeds 1.0% by mass, the Fe—Zn alloy layer is remarkably hardened and the workability is lowered. For example, cracks and peeling occur in the Fe—Zn alloy layer during wire drawing and cold rolling. Therefore, in the present invention, the Mg amount is in the range of 0.1% by mass to 1.0% by mass.

3) Other additive elements As additive elements other than Al and Mg, Ti, Li, Be, Na, K, Ca, Cu, La, Hf, Mo, W, Nb, Ta, Pb, Bi, Sr, V , Cr, Mn, Sn and the like can be added.

  Examples of elements that have an effect of improving corrosion resistance include Li, Be, Na, K, Ca, Cu, La, and Hf. Corrosion resistance is improved by adding 0.01 to 1.0% of one or more of these elements.

  Sr, V, Cr, Mn, and Sn have an effect of improving workability. When Sr etc. is added exceeding 0.5%, segregation becomes remarkable and it becomes easy to crack when processing a plated steel material. Therefore, when adding Sr etc., it is 0.5% or less.

4) Inevitable impurities Inevitable impurities are inevitably mixed into the plating layer from various elements in the plating line, particularly from the plating bath. Inevitable impurities include various substances such as those harmful to corrosion resistance, those harmful to workability, and those whose effects are unknown, such as C, P, S, O, and N.

5) Aspect ratio of α phase L / C: 2.0 or more The α phase is an Al-rich ZnAl alloy phase having a high Al concentration. The α phase is crystallized as primary crystals during the solidification process. This α phase contains almost no Mg. On the other hand, the β phase is a Zn single phase or a ZnMg alloy phase having a low Al concentration or almost no Al.

  In the present invention, in order to improve workability, the aspect ratio L / C of the length L in the axial direction of the steel wire to the length C in the cross direction perpendicular to the axis of the steel wire in the α phase is shown in FIG. Needs to be 2.0 or more. By setting the α phase aspect ratio L / C to 2.0 or more, the Fe—Zn alloy layer is not cracked or peeled off during wire drawing. In this way, the elongated α phase is formed by performing a surface reduction process having a large total area reduction rate, that is, a wire drawing process, a cold rolling process, or a skin pass (temper rolling).

  In the surface reduction processing with a large total area reduction rate, the true strain ε generated in the material is large. As the true strain ε increases, the aspect ratio L / C of the α phase also increases. For example, as shown in FIG. 4, when surface reduction processing with the true strain ε of 0.40 or more is performed, the aspect ratio L / C An α phase in which C is about 2.5 to 2.8 can be obtained. FIG. 4 shows that in order to obtain an aspect ratio L / C of 2.0 or more, a true strain ε of at least 0.40 or more is required.

6) Thickness of Fe—Zn alloy layer: 10 μm or less The thickness of the Fe—Zn alloy layer is preferably 10 μm or less. In order to adjust the thickness of the Fe—Zn alloy layer to 10 μm or less, it is necessary to control the immersion time of the steel wire in the plating bath, the temperature of the plating bath, and the composition of the plating bath within desired ranges, respectively. Of these, the immersion time of the steel wire in the plating bath is particularly important. The immersion time of the steel wire in the plating bath is preferably in the range of, for example, 3 seconds or more and 60 seconds or less (plating bath temperature: 450 ° C., plating bath composition; Mg = 0.1 to 1.0%, Al = 4-20%, balance Zn). This is because when the immersion time exceeds 60 seconds, the thickness of the Fe—Zn alloy layer exceeds 10 μm and becomes too thick. As shown in FIG. 8, when the average thickness of the Fe—Zn alloy layer is 10 μm or less, the workability of the plated steel wire is greatly improved. The workability was evaluated by a winding test of the same diameter steel wire described later.

7) Plating adhesion amount The average plating adhesion amount per unit area after wire drawing is preferably in the range of 150 to 400 g / m ² as a value converted in the case of the wire diameter φ1.91 mm. When the average plating adhesion amount is less than 150 g / m ² , the basic property of corrosion resistance is impaired. On the other hand, when the average plating adhesion amount exceeds 400 g / m ² , the pure plating layer cannot withstand the tensile stress, and the plating surface is cracked.

  Hereinafter, with reference to FIG. 2, the method for manufacturing the plated steel wire of this invention is demonstrated.

  First, a hard steel wire defined in JIS G3506 is prepared (step S1). The diameter of the prepared steel wire is φ5.0 mm and SWRH72A (average composition: 0.72% C-0.25% Si-0.45% Mn).

  This steel wire was subjected to surface reduction processing from an initial diameter of 5.0 mm to a finished diameter of 3.6 mm by wire drawing or cold rolling (step S2). In addition, the process of this process S2 is arbitrary in this invention, you may abbreviate | omit process S2 and may implement the following process S3 from process S1.

  Next, the surface-reduced steel wire is plated. A two-step plating method is used for the plating treatment. In the two-stage plating method, first, the steel wire was immersed and passed through a hot dip galvanizing bath or a hot dip galvanized aluminum plating bath containing Al at a concentration of 5% by mass or less to form a base plating layer (step S3). The plating bath temperature was about 450 ° C., and the immersion time of the steel wire in the plating bath was controlled in the range of 3 seconds to 60 seconds.

Subsequently, the steel wire plated with the base was immersed in a molten zinc aluminum / magnesium bath containing 0.1 to 1.0% by mass of Mg and 4 to 20% by mass of Al to form a ZnAlMg alloy plating layer (step) S4). The plating bath composition is Mg: 0.5%, Al: 9.5%, the remaining Zn, the plating bath temperature is about 450 ° C., and the immersion time of the steel wire in the plating bath is in the range of 3 seconds to 60 seconds. Controlled. As a result, a plating layer having a composition of Zn-11% Al-0.5% Mg was formed on the base plating layer. The average plating adhesion amount was 495 g / m ² .

  By subjecting the steel wire to the plating process by such a two-step plating method (steps S3 to S4), the final thickness of the Fe—Zn alloy layer could be suppressed to 10 μm or less.

  Next, the hot-rolled ZnAlMg alloy-plated steel wire was drawn, cold-rolled, or skin-passed (step S5). The hot-dip ZnAlMg alloy-plated steel wire was subjected to surface reduction by wire drawing, cold rolling or skin pass so that the total area reduction was 0.4 or more in terms of true strain ε. By such surface reduction processing, the aspect ratio L / C of the α phase was controlled to 2.0 or more. The wire drawing may be performed either dry or wet.

  Next, various examples will be described with reference to comparative examples.

<Α phase aspect ratio L / C measurement and plating layer structure>
The structures of the sample plating layers of Examples A and B and Comparative Examples C and D will be described with reference to FIG.

  Samples of Examples and Comparative Examples were prepared as follows.

Example A: Cold wire drawing 5.0 mm → 3.6 mm, 3.72 mm after plating, wire drawing 3.72 mm → 2.34 mm, ε = 0.93
Example B: Cold wire drawing 5.0 mm → 3.6 mm, 3.72 mm after plating, wire drawing 3.72 mm → 2.95 mm, ε = 0.46
Comparative Example C: Cold wire drawing 5.0 mm → 3.6 mm, 3.72 mm after plating, wire drawing 3.72 mm → 3.30 mm, ε = 0.24
Comparative Example D: Cold drawing 5.0 mm → 3.6 mm, 3.72 mm after plating, no drawing, ε = 0
The metallographic structure of the plating layer was observed over 3 fields of view with a scanning electron microscope (SEM).

  The structure of the sample plating layer of Example A is shown in FIG.

  The structure of the sample plating layer of Example B is shown in FIG.

  The structure of the sample plating layer of Comparative Example C is shown in FIG.

  The structure of the sample plating layer of Comparative Example D is shown in FIG.

  In each sample of Examples A and B, the aspect ratio L / C of the α phase was 2.0 or more.

  In the samples of Comparative Examples C and D, the α phase aspect ratio L / C was 2.0 or less.

  Further, ten α-phase grains were arbitrarily selected from each field of view of the SEM, the aspect ratio L / C was measured, and the average value was obtained. The relationship between the obtained average aspect ratio L / C and true strain ε is plotted in FIG.

  FIG. 4 is a characteristic diagram showing the relationship between the total area reduction ratio of the wire drawing and the aspect ratio, with the true strain ε on the horizontal axis and the aspect ratio L / C of the α phase on the vertical axis. It measured about the Zn-11.4% Al-0.32% Mg plating layer. As is clear from this figure, it was confirmed that the average aspect ratio L / C of the α phase exceeded 2.0 when the true strain ε was in the range of 0.40 or more.

<Processing evaluation method>
Next, a method for evaluating the hot-dip plating layer will be described with reference to FIG.
The workability of the plating layer was evaluated using a winding test method. In the winding test method, the plating wire having the same diameter is wound eight times around the outer periphery of the plating wire cut to an appropriate length (for example, 30 cm), and the appearance of the winding wire is as shown in FIGS. The score was divided into four stages and scored and evaluated.

<Evaluation results>
As shown to (a) of FIG. 5, the thing which does not confirm the crack which can be observed with an optical microscope over the whole sample was made into 3 points | pieces (highest score).

  As shown in (b) of FIG. 5, two samples with 10 or fewer cracks that could be confirmed with an optical microscope in the entire sample were used.

  As shown in (c) of FIG. 5, a sample in which more than 10 cracks in the entire sample can be confirmed with an optical microscope was taken as one point.

  As shown in (d) of FIG. 5, a sample in which large cracks or peelings in which one or more ground irons (base materials) were visible (by the naked eye) in the entire sample was recognized as 0 (no rating).

  FIG. 6 shows the results of the Zn-11.4% Al-0.32% Mg plating layer, with the aspect ratio L / C of the α phase on the horizontal axis and the winding test result (score) on the vertical axis. FIG.

  FIG. 7 shows the results of the Zn-11.5% Al-0.31% Mg plating layer, with the aspect ratio L / C of the α phase on the horizontal axis and the winding test result (score) on the vertical axis. FIG. This is a comparative example in which the thickness of the Fe—Zn alloy layer exceeds 10 μm, and it can be seen that the winding test score is poor even when the aspect ratio L / C of the α phase is 2 or more.

  In FIG. 8, the horizontal axis represents the alloy layer thickness (μm), the vertical axis represents the winding test result (score), and the Zn-11.5% Al-0.31% Mg plating layer (true strain ε = It is a characteristic view which shows the result of 0.46).

  Table 1 shows the results of examining wire drawing workability in Examples 1 to 5 and Comparative Examples 1 and 2 in which the Mg content was variously changed.

  In Examples 1 to 5, the wire drawing could be performed without causing cracks in the plating layer.

  In Comparative Example 1 (Mg: 1.95%), cracks occurred in the plating layer or the alloy layer in the die during wire drawing, and the wire could not be drawn in a healthy state.

In Comparative Example 2 (Mg: 1.10%), the drawing process itself is possible, but the whole is slightly defective.

<Corrosion test method>
The corrosion resistance of the plating layer was evaluated using a salt spray test specified in JIS Z2371.

<Evaluation results>
FIG. 9 is a characteristic diagram showing the results of a Zn-11% Al—xMg plating layer with the Mg amount on the horizontal axis and the corrosion test result (g / m ² ) on the vertical axis, with various Mg amounts. is there. A steel wire drawn in stages from an initial diameter of 3.2 mm to a finishing diameter of 2.14 mm was used as a test material. As can be seen from this figure, the corrosion resistance improves as the amount of Mg increases.

FIG. 10 is a characteristic diagram showing the results of a Zn-14.8% Al-0.59% Mg plating layer, with the true strain ε on the horizontal axis and the corrosion test result (g / m ² ) on the vertical axis. It is. In the figure, the characteristic plot line A is the result of molten Zn-14.8Al-0.59Mg plating (Example), the characteristic plot line B is the result of molten ZnAl plating (Comparative Example), and the characteristic plot line C is hot dip galvanized. The results of (Comparative Example) are shown. As is apparent from the figure, it can be seen that hot-dip ZnAlMg plating is superior in corrosion resistance to hot-dip ZnAl plating and hot-dip galvanizing.

  The hot dip galvanized steel wire of the present invention can be used for various applications such as wire for wire, aquaculture wire, fiber for concrete reinforcement, wire for bridge, PWS wire, PC steel wire, rope and spring. it can.

The figure which shows alpha phase typically in order to explain aspect ratio L / C of alpha phase. Process drawing which shows the manufacturing method of the hot dip galvanized steel wire of this invention. (A)-(d) is the SEM photograph which each shows the pure plating layer structure | tissue after a wire drawing process. The characteristic diagram which shows the correlation with aspect-ratio L / C of alpha phase in a Zn-Al-Mg alloy plating layer, and true distortion (epsilon). (A) Appearance photograph showing a sample with three evaluation points of the winding test, (b) Appearance photograph showing a sample with two evaluation points of the winding test, (c) Evaluation points of the winding test Is an appearance photograph showing a sample with one point, and (d) is an appearance photograph showing a sample with an evaluation score of 0 in the winding test. The characteristic line figure which shows the relationship between the aspect-ratio L / C of (alpha) phase in a Zn-Al-Mg alloy plating layer of the same component system as FIG. 4, and a winding test result. The characteristic diagram which shows the relationship between the aspect-ratio L / C of (alpha) phase in a Zn-Al-Mg alloy plating layer of another component system, and a winding test result. The characteristic line figure which shows the relationship between the alloy layer thickness in the Zn-Al-Mg alloy plating layer of the same component system as FIG. 7, and a winding test result. The characteristic diagram which shows the relationship between Mg content of an alloy plating layer, and corrosion weight loss. The characteristic diagram which shows the correlation with a true strain (epsilon) and corrosion weight loss by contrasting a Zn plating layer, a ZnAl alloy plating layer, and a ZnAlMg alloy plating layer.

Claims (2)

The average composition is mass%, Al: 4-20%, Mg: 0.10-1.00%, the remainder has a hot-dip plated layer made of Zn and inevitable impurities, and the hot-dip plated layer has a steel wire A hot-dip galvanized steel wire comprising an α phase having an aspect ratio L / C of 2.0 or more between the length L in the axial direction and the length C in the cross direction. The hot dip galvanized steel wire according to claim 1, wherein an average thickness of the Fe-Zn alloy layer formed between the hot dip plated layer and the steel wire is 10 m or less.

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