JP2013249492A - Hot-dip galvanized steel wire with excellent twisting characteristics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel wire which hardly causes vertical cracking to a twisting stress.SOLUTION: A hot-dip galvanized steel wire with excellent twisting characteristics contains, by mass, C: 0.8-1.05%, Si: 0.15-1.3%, Mn: 0.25-0.9%, Al: 0.01-0.08%, and N: 0.002-0.006%, and further contains, as needed, at least one selected from the group consisting of Cr: 0.5% or less, V: 0.1% or less, Nb: 0.1% or less, Mo: 0.3%, W: 0.5% or less, and B: 0.0025% or less, with the balance being iron and unavoidable impurities. In the steel wire, a steel portion has a pearlite structure subjected to drawing treatment, and the amount of hydrogen released at room temperature to 250°C when heating the steel is 0.15 ppm by mass or less.

Description

  The present invention relates to a high-strength galvanized steel wire having a tensile strength of 1960 MPa or more used for bridges, tethers, power transmission and distribution wires, and the like.

  A structural member such as a wire rope is manufactured by twisting or bundling a galvanized steel wire obtained by drawing a pearlite steel wire and applying a galvanizing treatment. From the viewpoint of reducing dead loads due to their own weight and shortening the work period, higher strength is required. However, the higher the strength of the galvanized steel wire, the easier it is to cause vertical cracks (delamination) when torsional deformation is applied during the manufacturing process. Once the vertical cracks occur, the fatigue strength and delayed fracture characteristics of the member Is known to deteriorate significantly, and there is a need for the development of a galvanized steel wire that has high strength and does not generate delamination.

As a technique for improving the delamination resistance characteristics of a steel wire, it is known that it is effective to reduce the amount of nitrogen in the steel, which is an embrittlement factor of the steel material. It is disclosed that a high-strength galvanized steel wire that does not cause delamination can be produced by controlling the amount of solute N to 10 ppm or less. However, in the examples, it is disclosed that a steel wire having a wire diameter of 4.9 mm and a strength of 216 kgf / mm ² can be formed by performing blueing without plating the steel wire. The manufacturing conditions are different.

  In the evaluation regarding the above delamination, a test is generally performed with a wire having a target length of 100 d (d: outer diameter [mm]) at a twisting speed of 20 to 60 rpm. FIG. 1 shows an example of the behavior of a torque curve during a twist test of a hot dip galvanized wire. In the twisting test of the plated wire, the process of propagation of the vertical crack generated at the initial stage of twisting is rare, and the fracture often occurs at the same time as the vertical crack occurs. Furthermore, in the case of a wire that has been hot dip galvanized, vertical cracks do not occur on the torque curve at the beginning of twisting. Often accompanied by cracks. The vertical crack is presumed to have occurred immediately before breakage. In order to improve the twisting characteristics of the wire and to ensure the safety of the structure, in addition to the delamination that occurs in the early stages of twisting, pay attention to the vertical cracks that are considered to be relatively difficult in manufacturing. There is a need.

JP-A-9-87803

  The present invention has been made paying attention to the above circumstances, and is a wire capable of suppressing the occurrence of delamination even with a high-strength hot-dip galvanized wire having a wire diameter of 4.0 mm to 7.0 mm and 1960 MPa or more. The purpose is to provide.

  The present inventors have made various studies on influencing factors on the twisting characteristics of high-strength galvanized steel wires. As a result, we focused on hydrogen in steel and studied the effect of hydrogen on the twisting characteristics, and obtained the following conclusion from the relationship between tensile strength (MPa) and diffusible hydrogen content (mass ppm) shown in FIG. It was.

  (1) Hydrogen may remain in the wire steel during casting and heating. These are hydrogen released when kept in a temperature range from room temperature to 250 ° C., and are generally called diffusible hydrogen that adversely affects mechanical properties. By performing the wire drawing process, this hydrogen is firmly trapped in the steel, and changes to a state called non-diffusible hydrogen which is difficult to escape even if kept at room temperature to 250 ° C.

  (2) Non-diffusible hydrogen trapped in the steel wire is subjected to hot dip galvanization, and in the surface layer part, the trap site disappears due to the thermal effect of the plating treatment, so that it becomes diffusible hydrogen again. Change. It was considered that this diffusible hydrogen acts on the concentration of torsional deformation during the torsion test and promotes vertical cracking. Further, in zinc used for plating, since hydrogen diffusion is very slow, it stays in the steel material for a long time, so that the twisting characteristic of the wire is deteriorated for a long time.

  That is, it has been found that if hydrogen is present in the plated steel wire, the twisting characteristics are deteriorated, and it is important to reduce the amount of hydrogen at the wire stage in order to improve the twisting characteristics.

The present invention is based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.8 to 1.05%,
Si: 0.15 to 1.3%
Mn: 0.25 to 0.9%,
Al: 0.01 to 0.08%,
N: 0.002 to 0.006%
In the hot dip galvanized steel wire, the balance of which is a steel component consisting of iron and inevitable impurities, the steel material portion other than the plating and the alloy layer has a pearlite structure, and the amount of diffusible hydrogen is 0. A high-strength hot-dip galvanized steel wire excellent in twisting characteristics, characterized by being 15 mass ppm or less.
(2) Furthermore, in mass%,
Cr: 0.5% or less,
V: 0.1% or less,
Nb: 0.1% or less,
Mo: 0.3%
W: 0.5% or less,
B: The high-strength hot-dip galvanized steel wire according to (1) above, which contains at least one selected from the group consisting of 0.0025% or less.

  According to the present invention, even with a high-strength hot-dip galvanized wire having a wire diameter of 4.0 mm to 7.0 mm and 1960 MPa or more, the occurrence of delamination can be suppressed.

It is a figure which shows an example of the behavior of the torque curve at the time of the twist test of a hot-dip galvanized steel wire. It is a figure which shows the change of the hydrogen-release profile by the relationship between the temperature (degreeC) and the hydrogen-release rate (mass ppm / min) by the temperature-programmed desorption analysis method in steel materials when a wire is made into a steel wire and a plated steel wire. . It is a figure which shows the relationship between the amount of hydrogen of the plated steel wire created in the Example, and a delamination generation rate.

  Below, the reason for limitation of the high-strength hot-dip galvanized steel wire excellent in twisting properties according to the present invention will be described in detail. First, the reasons for limiting the component composition of the steel wire of the present invention will be described below. In addition, “%” of the content of the component composition means “mass%”.

C: 0.80 to 1.05%
C has the effect of increasing strength by reducing the lamellar spacing and increasing the θ thickness. If it is less than 0.80% C, the wire strength becomes low. If it exceeds 1.05%, the drawability and the ductility of the wire are deteriorated, so the C content is set to 0.8 to 1.05%.

Si: 0.15 to 1.3%
Si is a deoxidizing element, a solid solution strengthening element for ferrite in pearlite, and also has the effect of suppressing strength reduction during hot dip galvanizing because it suppresses spheroidization of lamellar cementite during heat treatment. The lower limit of the amount required for the deoxidizer was specified as 0.15%. Further, when the content is 1.3% or more, the granulation of cementite in the drawn pearlite structure of the surface layer in the present invention is inhibited, so the Si content is set to 0.15% to 1.3%.

Mn: 0.25 to 0.9%
Mn is a strengthening element that increases the hardenability of steel and refines the lamellar spacing by slowing the pearlite transformation start time during patenting. In order to obtain the effect of increasing the wire strength, the content is set to 0.25% or more, and when it exceeds 0.9%, the transformation is delayed, and martensite is generated in the center segregation portion, thereby increasing the possibility that ductility is lowered. Therefore, the Mn content is set to 0.25 to 0.9%.

Al: 0.01 to 0.08%
Al is effective as a deoxidizing element. Moreover, nitride is formed and the austenite grain coarsening is suppressed. If the Al content is less than 0.01%, austenite grains become coarse during heating, and if it exceeds 0.08%, the cold workability due to Al ₂ O ₃ inclusions decreases, so the content is set to 0.08% or less. . Al is 0.01 to 0.08%, but is preferably 0.02 to 0.07%.

N: 0.002 to 0.006%
N forms AlN in the steel and suppresses coarsening of austenite during heating. In order to obtain this effect, 0.002% is necessary, and if the amount added is large, the strain aging during wire drawing adversely affects the wire drawing characteristics, so the upper limit was limited to 0.006%.

  The above is an essential element for obtaining the effect of the present invention, and a predetermined effect can be further obtained by adding an appropriate amount of the following elements.

Cr: 0.5% or less Cr refines the lamellar spacing of pearlite and improves the work hardening rate during wire drawing. In order to take advantage of the above action, addition of 0.1% or more is preferable, but excessive addition increases the pearlite transformation end time, generates martensite, and lowers the wire drawing characteristics, so that it is 0.5% or less. did.

V: 0.1% or less V forms fine carbonitrides in ferrite and contributes to an increase in strength. The above effect is preferably added at 0.02% or more. Excessive addition forms coarse carbonitride and lowers the ductility of the wire, so it was made 0.1% or less.

Mo: 0.3% or less Mo contributes to high strength because it suppresses pearlite transformation at high temperatures. Addition of 0.05% or more is preferable for effectively exhibiting the above action. On the other hand, when Mo is added excessively, the transformation end time becomes long, so the upper limit value was made 0.3%.

W: 0.5% or less In order to suppress the pearlite transformation, W is preferably added in an amount of 0.1% or more in order to effectively exhibit the above-described action contributing to the increase in strength. On the other hand, if W is added excessively, the transformation end time becomes long, so the upper limit was made 0.5%.

Nb: 0.1% or less Nb has an effect of suppressing coarsening of austenite grains during heating by generating carbonitride. Addition of 0.03% or more is preferable for effectively exhibiting the above action. On the other hand, when Nb is added excessively, the transformation end time becomes long, so the upper limit was made 0.1%.

B: 0.0025% or less When B exists in austenite as a solid solution B, it segregates at the grain boundary and suppresses the formation of proeutectoid ferrite. When the amount of B added is large, Fe ₂₃ (C, B) ₆ carbides precipitate at the grain boundaries, and the upper limit of the content is limited to 0.0025% in order to reduce ductility. In order to effectively exhibit the effect of addition of B, the content is preferably 0.0004% or more.

  The impurities P and S are not particularly limited, but are preferably 0.02% or less.

  Next, the reason for limiting the amount of hydrogen in the steel, which is important for improving the twisting characteristics of the high-strength galvanized steel wire intended in the present invention, will be described.

Diffusible hydrogen content in steel wire: 0.15 ppm or less (including 0)
In the present invention, the amount of hydrogen in the steel wire is defined as the amount of hydrogen measured by a temperature programmed desorption analysis method (TDS method) using a gas chromatograph. The profile shown in FIG. 2 shows the amount of release at each temperature when heated at a temperature elevation rate of 100 ° C./hr by the temperature programmed desorption analysis, and the area of the profile is the total amount of hydrogen released. As described above, the amount of diffusible hydrogen refers to the amount of hydrogen released from room temperature to 250 ° C. or less. When the amount of diffusible hydrogen exceeds 0.15 mass ppm, the risk of delamination increases in a high-strength plated steel wire of 1960 MPa or higher, so 0.15 ppm was made the upper limit. Preferably it is 0.12 ppm or less, More preferably, it is 0.10 ppm or less.

  In order to improve the strength of the wire efficiently, it is generally effective to draw a pearlite structure, and the present invention improves the ductility of such a drawn pearlite structure. is there.

Hereinafter, the present invention will be described more specifically with reference to examples.
Table 1 shows the chemical components of the examples (invention steel) and the comparative steel. The steel types shown in Table 1 were hot-rolled to 13.5 to 10.5 mm to produce a wire. In Table 2, DLP is obtained by direct patenting with molten salt after hot rolling, and LP is obtained by heating at 950 ° C. for 10 minutes and lead patenting at a temperature of 560-580 ° C.

  Thereafter, the wire was dehydrogenated by holding at room temperature or holding treatment in a heating furnace, and then the wire was drawn to 4.9 mm. Then, it was immersed for about 30 seconds in a 450 degreeC zinc bath, and the hot dip galvanized wire which has the intensity | strength of 1960-2060 MPa was manufactured.

  As for the hydrogen concentration, a 4.9 mm drawn wire was separately immersed in a 450 ° C. lead bath for 30 seconds, and stored in liquid nitrogen immediately after heat treatment or immediately after hydrogen analysis. This is a simple analysis because the hydrogen analysis in the plated steel wire is not easy to remove by plating, and the hydrogen amount and profile measured by thermal desorption analysis are almost the same as the steel wire after plating removal. It is.

  The twist value was defined as the number of times until breakage during the twist test in the distance between scores of 100d (d: diameter [mm]) length, and the average value of 20 samples. The twisting speed was 20 rpm. Table 2 shows the production conditions, hydrogen concentration, and mechanical properties.

  As shown in Table 2, sample no. 1 to 16 are high-strength galvanized steel wires having excellent twisting properties according to the present invention. Reference numerals 17 to 32 are comparative steel wires. Sample No. Although all of the comparative steel wires 17 to 32 were within the chemical composition range of the present invention, the amount of diffusible hydrogen was high, so delamination occurred in the torsion test at a strength level of 1960 MPa.

Claims (2)

% By mass
C: 0.8 to 1.05%,
Si: 0.15 to 1.3%
Mn: 0.25 to 0.9%,
Al: 0.01 to 0.08%,
N: 0.002 to 0.006%
In the hot dip galvanized steel wire, which is a steel component consisting of iron and inevitable impurities, the steel material portion has a pearlite structure that has been drawn, and the amount of diffusible hydrogen is 0.15 mass ppm or less A high-strength hot-dip galvanized steel wire with excellent twisting characteristics. Furthermore, in mass%,
Cr: 0.5% or less,
V: 0.1% or less,
Nb: 0.1% or less,
Mo: 0.3%
W: 0.5% or less,
The high-strength hot-dip galvanized steel wire according to claim 1, comprising at least one selected from the group consisting of B: 0.0025% or less.

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