JPH0987803A - Wire for hot dipped steel wire, excellent in longitudinal crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wire for hot dipped steel wire, having high strength and excellent in longitudinal crack resistance at the time of twisting. SOLUTION: This wire is a wire for hot dipped steel wire, composed of a steel which contains, by mass, 0.7-1.2% C, 0.5-2.0% Si, 0.2-2.0% Mn, 0.01-0.05% Al, and 30-150ppm N and in which the amount of solid-solution N is regulated to <=10ppm. Further, it is recommended that Cr is incorporated by 0.1-1.0% from the standpoint of the improvement of strength and workability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wire material for hot-dip galvanized steel wire having excellent resistance to vertical cracking. More specifically, it has a large diameter for use in manufacturing galvanized steel stranded wire, cables for suspension bridges and the like. The present invention relates to a wire rod for hot-dip galvanized steel wire (having a wire diameter of about 5 mm), which has high strength and is excellent in vertical crack resistance during twisting.

[0002]

2. Description of the Related Art In manufacturing PC steel wires and cables for suspension bridges, which require corrosion resistance, it is common to subject a high carbon steel wire rod to patenting, wire drawing, and then hot dip galvanizing. Target. According to this manufacturing method, the wire drawing improves the strength of the wire, but in the subsequent hot dip galvanizing process, the wire is heated to 400 ° C. or higher, so that the strength decreases. Moreover, as the strength is increased by wire drawing, the strength decrease due to plating increases, resulting in difficulty in increasing the strength of the plated steel wire.

Increasing the C content as a means for increasing the strength of the high carbon steel wire rod is considered to be the most industrially desirable method because it is inexpensive and highly effective. However, in the hyper-eutectoid region where the C content is 0.9% or more, brittle pro-eutectoid cementite is formed in a network form along the austenite grain boundaries during patenting. Therefore, during wire drawing, wire breakage occurs from the brittle proeutectoid cementite crack as a starting point, and wire drawability deteriorates. Moreover, there is no effect on the strength reduction due to plating, and the strength of the plated steel wire cannot be increased depending on the amount added.

On the other hand, Si has the effect of increasing the strength of the steel wire after the patenting treatment and also the strength of the steel wire after drawing, and improves the hardenability of the steel wire to precipitate pro-eutectoid cementite. Has the effect of suppressing. Further, it is possible to suppress the strength reduction due to plating and to enhance the strength of the hot dip plated steel wire. However, it is also known that the addition of a large amount of Si deteriorates the toughness and ductility of the steel wire. Therefore, in Japanese Unexamined Patent Publication No. 4-325627, S
A method for producing a steel wire having excellent ductility by adjusting the amount of i added is disclosed. However, this publication only mentions the characteristics of the drawn wire, not the characteristics of the plated steel wire.

As a technique for improving the twisting characteristic, it is known that it is effective to subject a steel wire to a bluing treatment, and for example, it is disclosed in Japanese Patent Laid-Open Nos. 4-246125 and 4-236742. Discloses a method in which, after hot-dip galvanizing, straightening processing or wire drawing processing is performed, and then bluing processing is performed. However, the processing after plating is liable to cause deterioration of corrosion resistance due to peeling of the plating layer, and for example, in the case of plated steel wire for suspension bridges, it is a method that should be avoided as much as possible to add processing to the steel wire after plating. .

It is also known that by releasing the tensile residual stress on the surface of the steel wire, vertical cracking at the time of twisting can be suppressed. For example, in Japanese Patent Publication No. 3-66386, a compressive residual stress is applied to the surface of the steel wire. The method of positively giving is disclosed. However, when the plating is performed after controlling the residual stress, the effect of the residual stress almost disappears due to the rise of the wire temperature during plating, and even if it is applied to the hot-dipped steel wire wire, the sufficient effect cannot be obtained. .

[0007] Thus, a hot-dip wire for a hot-dip steel wire which does not require special processing or heat treatment after hot-dip galvanizing, has high strength and is sufficiently excellent in vertical crack resistance during twisting. The wire rod for steel wire has not been developed.

[0008]

The present invention has been made in view of the above circumstances and has high strength and
An object of the present invention is to provide a wire material for hot-dip galvanized steel wire having excellent resistance to vertical cracking.

[0009]

[Means for Solving the Problems] The wire material for hot-dip galvanized steel wire of the present invention, which has achieved the above object, is C: 0.7 in mass%.
~ 1.2%, Si: 0.5-2.0%, Mn: 0.2-
2.0%, Al: 0.01 to 0.05%, N: 30 to 1
The gist is that it is made of steel containing 50 ppm and the amount of solute N in the steel is 10 ppm or less. Further, it is recommended to contain Cr in an amount of 0.1 to 1.0% from the viewpoint of improving strength and wire drawing workability.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have found that as a result of research focusing on trace chemical components in steel, especially the amount of solute N, the effect on longitudinal crack resistance is extremely large. Was conceived. In the present invention, the amount of solute N is calculated from the difference between the total amount of N in steel measured by chemical analysis and the amount of N in AlN obtained by the extraction residue method.

It is known that the amount of solute N affects the characteristics of the steel wire. For example, the inventors of the present invention disclosed in Japanese Patent Laid-Open No. 6-27.
1937 discloses that solid solution N accelerates aging and deteriorates the toughness of the steel wire. However, there has been no disclosure up to now of the amount of solute N and the vertical cracking property of the hot dip plated steel wire.

The resistance to vertical cracking depends on the wire diameter, and the thinner the wire diameter, the higher the strength without causing vertical cracking. A steel cord for tires is an example, and a high strength material of 300 kgf / mm ² or more is obtained with a wire diameter of about 0.2 mm. However, the strength of about 200 kgf / mm ² is the limit for thick wire (for example, wire diameter of about 5 mm), and
If it exceeds 00 kgf / mm ² , vertical cracking will occur during twisting. The present inventors have found that the adverse effect of the presence of solute N on the vertical cracking resistance is more pronounced in a thick wire, and the amount of solute N is 10
It has been found that the upper limit of the strength (vertical cracking limit tensile strength) at which vertical cracking does not occur at the time of twisting is extremely reduced when the content exceeds ppm. Therefore, in the present invention, the upper limit of the amount of solute N is set to 10 ppm.

The reasons for limiting the components of the present invention will be described below. C is an effective and economical element for increasing the strength, and as the C content is increased, the strength after patenting, the work hardening amount during wire drawing, and the strength after wire drawing increase. Therefore, in the present invention, the lower limit is set to 0.7%. However, if the amount of C is too large, the precipitation of pro-eutectoid cementite cannot be prevented, so the upper limit of the amount of C was made 1.2%.

Si is an element required as a deoxidizing agent, and also forms a solid solution with ferrite, and exhibits a remarkable effect of strengthening the solid solution. Further, Si in ferrite has an effect of reducing the strength reduction due to the bluing treatment or the hot dip galvanizing treatment after wire drawing, 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 wire drawing decreases, so the upper limit was made 2.0%.

Mn is also an element required as a deoxidizer, and is effective in improving the hardenability of steel and improving 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, segregated portions of Mn are formed and a supercooled structure such as martensite or bainite is formed to deteriorate the wire drawing workability, so the upper limit is 2.0%.
And

Al is effective as a deoxidizing agent and also effective for preventing coarsening of austenite grain size. Moreover, since it also has the effect of reducing the solute N that is harmful to the vertical cracking resistance by combining with N in the steel, 0.02% or more is necessary. However, even if added excessively, the effect is saturated and it becomes a factor that impairs the economical efficiency, so 0.05% was set as the upper limit.

N becomes a nitride of Al in steel and is effective in preventing coarsening of the austenite grain size during heating. The effect is not sufficiently exhibited when the added amount is 30 ppm or less, so the lower limit was set to 30 ppm. On the other hand, if added excessively, not only does the amount of Al nitride increase excessively, which adversely affects the wire drawability, but also the amount of solute N increases, which adversely affects the vertical cracking resistance of the steel wire, so the upper limit is made 150 ppm. There is a need.

The wire material for hot-dip steel wire of the present invention may contain Cr in an amount of 0.1 to 1.0% in addition to the above components. Cr is effective in reducing the lamellar spacing of pearlite and improves the strength and wire drawing workability of the wire. In order to obtain such effects, it is necessary to add 0.1% or more. However, if it is too much, the transformation end time will be too long,
The upper limit is set to 1.0%, because it causes an increase in the size of equipment and a decrease in productivity.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made in view of the gist of the preceding and the following. It is included within the technical scope of.

[0020]

Example Steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace, and hot rolled into a steel wire having a wire diameter of 11 mm, followed by lead patenting treatment (condition: reheating 950 ° C. × 5 minutes , A constant temperature state of 540 ° C. for 4 minutes). The obtained steel wire was continuously drawn to a target wire diameter of 4.9 mm with a surface reduction rate of 80.2%. The number of dies at this time was 7, and the wire temperature was maintained at 170 ° C. or lower by cooling the wire at the outlet of any die.

Using the wire thus obtained, the limit tensile strength for longitudinal cracking was measured by the following method. The drawn steel wire, 20
Tensile test and twisting test were performed using a steel wire that was heated (blueing) for 15 minutes in the temperature range of 0 to 450 ° C (interval of 10 ° C) and vertical cracking occurred during the twisting test. The tensile strength at which the heating temperature disappears was defined as the longitudinal crack limit tensile strength. For example, FIG. 1 is a graph schematically showing a method for calculating the above-mentioned vertical cracking limit tensile strength, and shows that vertical cracking occurred in the twist test in the heat treatment in the range of 100 to 400 ° C. On the other hand, in the tensile test, it can be seen that the heating temperature shows the maximum value at a heating temperature of 200 ° C., but in the twisting test, around 200 ° C. is within the region where vertical cracking occurs, and vertical cracking does not 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 and the above-mentioned longitudinal cracking limit tensile strength.

The method for measuring the amount of solute N is to use a drawn steel wire, first to measure the total amount of N by chemical analysis, and then to analyze the amount of AlN by the extraction residue method to combine it with Al. The amount of N contained is determined, and the difference (-) is defined as the amount of solid solution N. When analyzing the amount of AlN by the above extraction residue method,
First, a steel wire was melted using a 10% acetylacetone-based electrolytic solution, and the resulting 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 the neutralization titration method.

[0023]

[Table 1]

No. Reference numerals 1 and 2 are examples of the present invention, and the vertical crack limit tensile strength shows a high value. No. Nos. 3, 5 and 7 are comparative examples in the case where C, Si and Mn are too small, and the longitudinal crack limit tensile strength shows a low value. No.
No. 4 is a comparative example when the amount of C was too large, and the amount of pro-eutectoid cementite was large and the wire drawability was deteriorated, resulting in disconnection during wire drawing. No. No. 6 is a comparative example in the case where the amount of Si is too large, the toughness of the LP wire rod was lowered, and the wire was broken during wire drawing. No. 8 is a comparative example when Mn is too much,
Due to the existence of supercooled tissue, the wire was broken during wire drawing. N
o. 9 is a comparative example when Al is too small,
The precipitation amount of N was insufficient and the crystal grain size of the LP material was coarsened, resulting in disconnection during wire drawing. No.
No. 10 is a comparative example when the total amount of N is too small, and
The precipitation amount of N was insufficient and the crystals of the LP material became coarse, resulting in disconnection during wire drawing. No. 11
Is a comparative example in the case where the total amount of N is too large and the amount of solute N exceeds the range of the present invention, and the limit tensile strength for longitudinal cracking is low. No. Nos. 12 and 13 are comparative examples in the case where the amount of solute N exceeds the range of the present invention and is too large, and the longitudinal crack limit tensile strength is low.

No. Nos. 14 to 17 are examples containing Cr, and No. Since Nos. 14 and 15 satisfy all the conditions of the present invention, the vertical cracking limit tensile strength shows a high value. N
o. No. 16 is a comparative example in the case where the amount of solute N exceeds the range of the present invention and is too large, and the limit tensile strength for longitudinal cracking is low. No. 1
No. 7 is a comparative example in the case where the amount of Cr is too large. It takes a long time to finish the transformation, the transformation does not finish during lead patenting, and there is a supercooled structure. Therefore, the wire breaks during wire drawing. .

FIG. 2 is a graph in which the results of Table 1 are arranged by the amount of solid solution N and the limit tensile strength of longitudinal cracking. From the graph of FIG. 2, it is understood that the amount of solute N should be controlled to 10 ppm or less in order to obtain high strength without causing vertical cracks during twisting.

[0027]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, it is possible to provide a wire rod for hot-dip galvanized steel wire which has high strength and is excellent in longitudinal crack resistance.

[Brief description of drawings]

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

FIG. 2 is a graph showing the relationship between the amount of solid solution N and the limit tensile strength of longitudinal cracking.

Claims (2)

[Claims] 1. C: 0.7 to 1.2%, S in mass%
i: 0.5 to 2.0%, Mn: 0.2 to 2.0%, A
1: 0.01 to 0.05%, N: 30 to 150 ppm
A wire rod for hot-dip galvanized steel wire, which is excellent in vertical cracking resistance, characterized in that it comprises the balance Fe and unavoidable impurities, and that the amount of solute N in the N is 10 ppm or less. 2. The wire material for hot-dip galvanized steel wire according to claim 1, further containing 0.1 to 1.0% of Cr.