JPH11106830A - Heat treatment of steel wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form fine pearlitic structure causing to the improvement of strength by executing the cooling while electrically conducting to a heated steel wire. SOLUTION: The objective wire rod is a steel cord, PC steel wire, piano wire, or the like particulary needed to the strength. As the current for electrically conducting, DC, AC or the current pulsatively conducting these is mensioned, but the AC is desirable. The conducted current density is desirable to be about 5 A/mm<2> in the case of continuously conducting because the cooling is easily executed. In the case of discontinuously conducting, about 10 A/mm<2> current density can be adopted. In the case of using the air as a coolant in a patenting, the current density is gradually lowered accompanied with the cooling of the steel wire. For example, firstly, the electric conduction is executed at 5 A/mm<2> and gradually lowered, and at the time of becoming about 2.8 A/mm<2> , the steel wire is made to about 600 deg.C, and held for about 30 sec, and further, the current density is lowered to cool the steel wire and finally, the current is stopped to cool the steel wire to the room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat treatment method for obtaining a high-strength steel wire.

[0002]

2. Description of the Related Art Conventionally, when manufacturing a piano wire such as a spring steel wire, a patenting process using molten lead or a fluidized bed as a cooling medium has been performed. The purpose of this heat treatment is to obtain a high-strength steel wire as a fine pearlite structure, and for this purpose, various heat treatment conditions have been proposed. Generally, to obtain a finer pearlite structure, it is necessary to perform a patenting treatment at a lower temperature.

[0003]

However, if the temperature is lowered too much by the conventional heat treatment method, bainite is formed.
There was a problem that the strength of the obtained steel wire was reduced.

Accordingly, an object of the present invention is to provide a heat treatment method capable of obtaining a high-strength steel wire having a finer pearlite structure even when transformed at the same temperature as in the conventional heat treatment.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above-mentioned object, and the feature of the present invention is to perform patenting while energizing. That is, the steel wire is heated to a predetermined temperature and energized when it is cooled.

Here, the target wire is particularly required to have high strength, for example, a piano wire such as a steel cord, a PC steel wire, or a spring steel wire.

The cooling medium for patenting includes air, salt, fluidized bed, and lead. When lead is used as the cooling medium, the steel wire may be covered with an appropriate insulator to perform patenting.

The current to be supplied may be direct current, alternating current, or a current flowing in a pulsed manner. The reason why a fine pearlite structure can be obtained by energization is not clear, but tests described later have confirmed that good results can be obtained by energization of alternating current.

[0009] The current density to be supplied, when continuously energized, about 5A / mm ² preferably facilitate cooling. When current is intermittently applied, the current density may be about 10 A / mm ² . In the case where air is used as a cooling medium for patenting, the current density may be gradually reduced as the steel wire is cooled. The cooling rate can be adjusted by lowering the current density or changing the non-energizing time when the power is intermittently energized.

[0010]

Embodiments of the present invention will be described below. (Test Example 1) A steel wire obtained by using a 4 mmφ steel wire containing 0.80 wt% C and performing a patenting process while energizing, and performing a patenting process without energizing. Was observed with a SEM, and the hardness of each steel wire was measured.

The patenting was performed by heating the steel wire to 1000 ° C. and air cooling. Among them, in the embodiment, a current having a current density of 5 A / mm ² is supplied to the steel wire during air cooling, and the current density is gradually reduced. In the cooling process, when the current density is about 2.8 A / mm ² , the steel wire is kept at about 600 ° C. and held for about 30 seconds, and the current density is further lowered to cool the steel wire. Then, the current was finally stopped and the temperature was cooled to room temperature. On the other hand, in the comparative example, no current was applied, and air cooling was performed at a higher speed than in the example.

FIG. 1 shows the metal structure of the obtained steel wire.
(A) shows an example and (B) shows a comparative example.
In the figure, white portions indicate cementite, and black portions indicate ferrite, and it is apparent that the example has a finer structure. The pearlite lamella spacing in the example was about 0.09 μm.

As a result of the hardness measurement, the comparative example shows 327.
In contrast to 5 Hv, the example was excellent in hardness of 368.9 Hv, indicating high strength.

(Test Example 2) Next, the same test as in Test Example 1 was performed while applying an alternating current in a pulsed manner. The wire used here is the same as in Test Example 1. An example in this test is that when cooling, an alternating current is supplied at a current density of 10 A / mm ^2.
, And the cooling was performed by increasing the time during which no current was supplied. On the other hand, in the comparative example, current was not applied at all, and cooling was performed at a higher speed than in the example. The cooling was air-cooled in both the examples and comparative examples. Then, the structure was observed and the hardness was measured.

FIG. 2 shows the metal structure of the obtained steel wire.
(A) shows an example and (B) shows a comparative example.
Also in this test example, it can be seen that the example has a finer structure. Here also, the pearlite lamella spacing in the example is about 0.09 μm, which is an extremely fine structure.

As a result of the hardness measurement, the comparative example is 325.
In contrast to 9 Hv, the example was superior in hardness to 365.7 Hv, indicating high strength.

In the above test, the case where an alternating current is applied is taken as an example, but it is expected that similar results can be obtained even if a direct current is applied.

[0018]

As described above, according to the method of the present invention, it is possible to obtain a high-strength steel material having a fine pearlite structure by performing patenting while energizing. In particular, in the method of the present invention, lead does not necessarily need to be used as a cooling medium, which is also preferable from an environmental viewpoint. Therefore, P
The method of the present invention can be effectively used in fields where strength is required, such as piano wire such as C steel (wire) and steel cord.

[Brief description of the drawings]

FIG. 1A is a micrograph showing the metallographic structure of a steel wire subjected to patenting while applying an alternating current, and FIG. 1B is a micrograph showing the metallographic structure of a steel wire patented without applying an electric current. It is.

FIG. 2 (A) is a micrograph showing the metallographic structure of a steel wire subjected to patenting while applying an AC pulse, and (B)
3 is a photomicrograph showing the metallographic structure of a steel wire patented without energization.

Claims (2)

[Claims] 1. A heat treatment method for a steel wire to be patented, wherein, when performing the patenting, cooling is performed while energizing a heated steel wire. 2. The method for heat treating a steel wire according to claim 1, wherein an alternating current is applied.