JPH11209847A - Wire rod for hot-dip metal coated steel wire, excellent in longitudinal crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wire rod for hot-dip metal coated steel wire, capable of wire drawing even if patenting treatment for rolled stock is omitted, having high strength, and exibiting excellent longitudinal crack resistance. SOLUTION: In the wire rod for hot-dip metal coated steel wire, the maximum grain size of second-phase ferrite existing in the position at a depth of 1/16 to 1/4 the diameter from the outside periphery of the cross section of the wire rod is regulated to <=107 μm. Moreover, it is desitable to incorporate 0.01-0.05% Al and 30-150 ppm N and also to regulate the amount of solid solution N, among N, to <=10 ppm. Further, it is more desirable that respective contents of Al and N satisfy Al/N>=4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention can directly draw a rolled material without a patenting process,
In addition, the present invention relates to a hot-dip galvanized steel wire having excellent longitudinal cracking resistance. More specifically, the present invention relates to a large-diameter (wire diameter of 5 mm) wire used for manufacturing a galvanized steel stranded wire, a suspension bridge cable, and the like.
The present invention relates to a wire rod for a hot-dip coated steel wire having a high strength and a high resistance to longitudinal cracking during twisting.

[0002]

2. Description of the Related Art In the manufacture of PC steel wires and cables for suspension bridges that require corrosion resistance, it is common to apply a patenting process to a high-carbon steel wire, draw it, and then apply galvanizing. It is a target.

As a technique for improving the torsion characteristics of a hot-dip galvanized steel wire, it is known that a bluing treatment is effective on a steel wire.
JP-A-46125 and JP-A-4-236742 disclose a method in which after hot-dip plating is performed, straightening or drawing is performed, and then bluing is performed. However, processing after plating is likely to cause peeling of the plating layer and cause deterioration of corrosion resistance. For example, in the case of a plated steel wire for a suspension bridge, it is a method to avoid processing the plated steel wire as much as possible. .

[0004] It is also known that by releasing the residual tensile stress on the surface of the steel wire, it is possible to suppress longitudinal cracking during twisting. For example, Japanese Patent Publication No. 3-66386 discloses that the surface of the steel wire has a compressive residual stress. Is disclosed. However, when plating is performed after controlling the residual stress, the effect of the residual stress almost disappears due to an increase in the wire temperature during plating, and a sufficient effect is not obtained even when applied to a wire for hot-dip coated steel wire. .

Further, conventional wire rods for hot-dip galvanized steel wires are subjected to wire drawing after being subjected to a patenting process. Wires which are drawn without a patenting process have been put to practical use. Did not.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and can be drawn without applying a patenting treatment to a rolled material, and has high strength and excellent longitudinal crack resistance. An object of the present invention is to provide a wire rod for a hot-dip galvanized steel wire exhibiting the following.

[0007]

A wire rod for a hot-dip coated steel wire according to the present invention which has achieved the above object is a second phase ferrite present at a position 1/16 to 1/4 of the diameter from the outer periphery of the wire cross section. Is intended to have a maximum particle size of 10 μm or less, and Al: 0.01 to 0.05% (% by mass,
The same shall apply hereinafter), N: 30 to 150 ppm,
Of these, the amount of solute N is preferably 10 ppm or less, and more preferably, the content of Al and N satisfies Al / N ≧ 4.

Further, as elements other than Al and N, C: 0.
7 to 1.2%, Si: 0.5 to 2.0%, Mn: 0.2
To 2.0%, the balance being Fe and inevitable impurities, and further containing 0.1 to 1.0% of Cr.
It is recommended from the viewpoint of improving strength and drawability.

In the present invention, the solute N content is a value calculated from the difference between the total N content in steel measured by chemical analysis and the N content in AlN obtained by the extraction residue method. .

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The vertical cracking resistance has a wire diameter dependence, and the thinner the wire diameter, the higher the strength without the occurrence of vertical cracks. For example, in the case of a tire steel cord, the wire diameter is as thin as 0.2 mm, so it is 300 kgf / mm.
^Two or more high-strength materials have been obtained. On the other hand, for a wire having a large diameter (for example, a wire diameter of about 5 mm), about 180
kgf / mm ² is the upper limit of strength, about 180 kgf / mm ²
If it exceeds, vertical cracks will occur at the time of twisting.

The present inventors have found that the presence of dissolved N has an adverse effect on the longitudinal cracking resistance of a wire, and this adverse effect is particularly remarkable for a large diameter wire, and the amount of dissolved N exceeds 10 ppm. It is found that the upper limit of strength (vertical crack limit tensile strength) at which vertical cracks do not occur during twisting is extremely reduced, and the amount of solid solution N in steel is set to 10 ppm or less to achieve excellent resistance. A wire rod for hot-dip galvanized steel wire exhibiting vertical cracking properties was previously filed (JP-A-9-87803). However,
The wire for the hot-dip galvanized steel wire is used for wire drawing.
It was assumed that a patenting process was performed. After that, the present inventors conducted intensive studies and found that C was 0.7%.
Even in the eutectoid steel or hypereutectoid steel contained above, ferrite exists as the second phase in the first phase such as pearlite, and by controlling the grain size of the second phase ferrite, the patenting is performed. The present inventors have found that a wire rod for hot-dip coated steel wire having high strength and excellent longitudinal crack resistance can be obtained without performing any treatment, and conceived the present invention.

The second phase ferrite acts as an internal defect during twisting, and a vertical crack is generated starting from the second phase ferrite. As shown in FIG. 2, the results of the examples described below are arranged in terms of the amount of solute N and the critical tensile strength of longitudinal cracks. When the maximum grain size of the second phase ferrite is 10 μm or less, the amount of solute N is reduced. When the maximum grain size of the second phase ferrite exceeds 10 μm, even if the amount of solute N is 10 ppm or less, the longitudinal crack limit tensile strength is greatly improved by setting the tensile strength at 10 ppm or less. It can be seen that the degree hardly improves. In particular, 180 kgf / m
To prevent longitudinal cracking of Zn-plated steel wire having a tensile strength exceeding m ² , the maximum grain size of the second phase ferrite should be 10 μm.
m needs to be controlled. The region where the vertical cracks occur is mainly located at 1/16 to 1/4 of the diameter from the outer periphery in the cross section of the rolled wire rod. Therefore, the maximum grain size of the second phase ferrite in this region is 1%.
It is very effective to control it to 0 μm or less. Less than,
The reasons for limiting the components of the present invention will be described.

C is an effective and economical element for increasing the strength. As the C content increases, the amount of work hardening during drawing and the strength after drawing increase. Therefore, the lower limit of the C content was set to 0.7%. However, if the amount of C is too large, precipitation of proeutectoid cementite cannot be prevented.
The upper limit of the amount was 1.2%.

[0014] Si is an element necessary as a deoxidizing agent, and also forms a solid solution with ferrite and exerts a remarkable effect on solid solution strengthening. Further, Si in ferrite has an effect of reducing a decrease in strength due to hot-dip galvanizing, and is an essential element for producing a high-strength steel wire. Therefore, the lower limit was set to 0.5%. However, if added excessively, the ductility of the steel wire after drawing is reduced, so the upper limit was set to 2.0%.

Mn is also an element necessary as a deoxidizing agent, and is effective in improving the hardenability of steel and increasing the uniformity of the structure in the cross section of the steel wire. Therefore, the lower limit is 0.2%
Set to. However, if the amount of Mn is too large, a segregated portion of Mn is formed, and a supercooled structure such as martensite or bainite is generated to deteriorate wire drawability, so the upper limit is 2.0%.
And

Al is effective as a deoxidizing agent and is also effective in preventing austenite grain size from becoming coarse. Moreover, it has an effect of reducing solid solution N which is harmful to longitudinal cracking resistance by bonding with N in steel, so that 0.01% or more is required. However, even if it is added excessively, the effect is saturated and the economy is impaired. Therefore, the upper limit is set to 0.05%.

N becomes a nitride of Al in the steel and is effective in preventing the austenite grain size from becoming coarse during heating. If the amount is 30 ppm or less, the effect is not sufficiently exhibited, so the lower limit is set to 30 ppm. On the other hand, if added in excess, not only does the amount of Al nitride increase too much, which adversely affects drawability, but also increases the amount of solute N and adversely affects the longitudinal cracking resistance of the steel wire, so the upper limit is 150 ppm. There is a need.

Further, the content of Al and N is Al / N ≧ 4.
Is very effective in reducing the solute N content to 10 ppm or less and increasing the longitudinal tensile limit tensile strength. FIG.
It is a graph which shows the relationship of the value of Al / N of a wire rod for hot-dip coating steel wires and the critical tensile strength of a vertical crack, and when the maximum grain size of a 2nd phase ferrite is 10 micrometers or less, when Al / N is less than 4, It can be seen that the critical tensile strength at longitudinal cracks is small, and the critical tensile strength at longitudinal cracks is large when the value of Al / N is 4 or more. The atomic weight of Al is 26.98 and the atomic weight of N is 1
Since it is 4.0, the atomic weight ratio of Al / N is 1.93. Therefore, if the ratio of Al / N in steel is 1.93,
Theoretically, all N is combined with Al to form AlN, but it takes a considerably long time to reach an equilibrium state, and mass production is difficult. As is clear from the graph of FIG. 3, the critical tensile strength at longitudinal cracks is remarkably increased when the value of Al / N is 4 or more. It is possible to sufficiently reduce the vertical cracking resistance by sufficiently reducing it.

In the wire rod for a hot-dip coated steel wire of the present invention, Cr may be contained in an amount of 0.1 to 1.0% in addition to the above components. Cr is effective in reducing the lamella spacing of pearlite and improves the strength and drawability of the wire.
It is desirable to add 0.1% or more. However, if it is too large, the transformation end time becomes too long, which leads to an increase in the size of the equipment or a decrease in productivity. Therefore, the upper limit is preferably set to 1.0%.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any change in the design based on the gist of the preceding and following aspects will be described. Are included within the technical scope of

[0021]

EXAMPLE Steel having the chemical components shown in Table 1 was melted in a vacuum melting furnace, and a steel wire having a wire diameter of 11 mm was produced by hot rolling.
At this time, in order to control the grain size of the second phase ferrite in the cross section of the wire, the mounting temperature during rolling is set to 700 to 9
Change the temperature in the range of 50 ° C, and
Cooling rate of wire rod to 1 ℃ / sec from 1 ℃ / sec to 10 ℃ / sec
Changed.

The obtained steel wire was continuously drawn at a reduction in area of 80.2% to a target wire diameter of 4.9 mm. At this time, the number of dies was 7, and the temperature of the wire was maintained at 180 ° C. or lower by cooling the wire at the exit of each die. Using the obtained wire, the critical tensile strength of a vertical crack was measured by the following method.

The drawn steel wire is subjected to a heating (blueing) treatment in a temperature range of 200 to 450 ° C. (interval of 10 ° C.) for 15 minutes and a tensile test and a torsion test are performed using the air-cooled steel wire. The tensile strength at the heating temperature at which vertical cracks did not occur during the round test was defined as the critical tensile strength of vertical cracks. For example, FIG. 1 is a graph schematically showing a method for calculating the above-described vertical crack limit tensile strength, and shows that a vertical crack has occurred in a twist test in a heat treatment in a range of 100 to 400 ° C. On the other hand, in the tensile test, it can be seen that the heating temperature is 200 ° C., and the tensile strength shows the maximum value. However, in the torsion test, around 200 ° C. is in the region where vertical cracks occur, and no vertical cracks occur. The tensile strength in the region shows the maximum value at 400 ° C. In the present invention, this value is defined as the longitudinal crack limit tensile strength.

Table 1 also shows the chemical composition of the steel wire, the grain size of the second phase ferrite, and the critical tensile strength of the vertical crack.

The method of measuring the amount of solute N is to use a drawn steel wire, first measure the total amount of N by chemical analysis, and then analyze the amount of AlN by the extraction residue method to combine with the Al. The difference (-) was determined as the solid solution N amount. In analyzing the amount of AlN by the above extraction residue method, first, a steel wire was dissolved using a 10% acetylacetone-based electrolyte, and the obtained solution was subjected to absorption filtration with a filter having a pore size of 0.2 μm to extract a residue. Using this residue, the amount of AlN was determined by a neutralization titration method.

The maximum grain size of the second phase ferrite can be determined by observing a region of 1/16 to 1/4 of the diameter from the outer periphery of the cross section of the wire using an electron microscope. In this case, the area is measured using an image analyzer and converted into the diameter of a circle having an equivalent area.

[0027]

[Table 1]

No. 1-3,6,18,19,21,2
Nos. 7 and 28 are examples of the present invention, and have a high value for the longitudinal crack limit tensile strength.

No. Nos. 4, 5, 7, 20, and 22 are comparative examples in which the ferrite grain size is too large, and all have low longitudinal tensile limit tensile strength.

No. 8, 10, 12 are C, S respectively
This is a comparative example in which i and Mn are too small, and shows a low value of critical tensile strength in longitudinal cracks. No. No. 9 is a comparative example in which the amount of C is too large, the amount of proeutectoid cementite is large, and the drawability is deteriorated, and as a result, the wire is broken during drawing. No. 11 is a comparative example in which Si is too much,
The toughness of the wire was reduced, and the wire was broken during drawing. No. 13
Is a comparative example when Mn is too large, and the wire was broken during drawing due to the existence of a supercooled structure. No. 14 is
This is a comparative example in which Al is too small, where the amount of AlN deposited is insufficient and austenite grains are coarsened,
As a result, the wire was broken during wire drawing. No. 15 is a comparative example in which the total N amount is too large and the solute N amount exceeds the range of the present invention, and the longitudinal crack limit tensile strength is low. No. 24 is a comparative example in which the amount of Cr is too large, and the wire was broken during the drawing. In addition, No. 14-17,2
Nos. 3, 25 and 26 are comparative examples in which the amount of solute N exceeds the range of the present invention and is too large, and the longitudinal tensile limit tensile strength is low.

[0031]

Since the present invention is constructed as described above, it can be drawn without applying a patenting treatment to a rolled material, and is a hot-dip coated steel wire having high strength and excellent vertical cracking resistance. It has become possible to provide a wire for hot-dip coated steel wire which does not require special processing or heat treatment after hot-dip plating for the purpose of improving longitudinal cracking.

[Brief description of the drawings]

FIG. 1 is a graph schematically showing a method of calculating a critical tensile strength of a vertical crack.

FIG. 2 is a graph showing the relationship between the amount of solute N and the critical tensile strength of a longitudinal crack.

FIG. 3 is a graph showing the relationship between the value of Al / N and the critical tensile strength of a vertical crack.

Claims (5)

[Claims] An excellent longitudinal cracking resistance characterized in that the maximum grain size of the second phase ferrite present at a position 1/16 to 1/4 of the diameter from the outer periphery of the cross section of the wire is 10 µm or less. Wire rod for hot-dip coated steel wire. 2. Al: 0.01-0.05%, N: 30
2 to 150 ppm, wherein the amount of solute N in the N is 10 ppm or less.
The wire rod for hot-dip galvanized steel wire according to 1. 3. The hot-dip steel wire according to claim 2, wherein the contents of Al and N satisfy Al / N ≧ 4. 4. C: 0.7-1.2%, Si: 0.5-
2.0%, Mn: 0.2 to 2.0%, with the balance Fe
The hot-dip galvanized steel wire according to any one of claims 1 to 3, comprising a unavoidable impurity. 5. The hot-dip steel wire according to claim 1, further comprising 0.1 to 1.0% of Cr.