Wire rods with superior drawability and manufacturing method...

Details Wire rods with superior drawability and manufacturing method... Wire rods with superior drawability and manufacturing method...

1999-10-15

2001-07-24

Yee, Deborah (Department: 1742)

Metal treatment

Stock

Ferrous

C148S330000, C148S333000, C148S598000

Reexamination Certificate

active

06264759

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing wire rods and steel wire for use in making bead wires, wire ropes and springs. More specifically, the present invention relates to high drawability wire rods and a manufacturing method therefor, in which wire drawing is possible without carrying out a patenting (an intermediate heat treatment during the drawing).
DESCRIPTION OF THE PRIOR ART
Generally, in making wire rods as a raw stock for bead wires, wire ropes and springs having high strength, two methods have been proposed in which wire drawing is carried out on the wire rods while raising the strength of the rods, or the drawing strength due to the work hardening phenomenon during the wire drawing is utilized. These are the major two methods which are currently used. However, the raising of the strength is accompanied by the lowering of the ductility, and therefore, a patenting has to be carried out before reaching the target wire diameter. On the other hand, if the strength of the steel wire is improved by increasing the drawing strain, there is the advantage that the patenting can be skipped, but delamination is liable to occur, thereby making it difficult to secure the high strength.
Specifically, in most of the conventional techniques, in order to improve the drawability of a carbon steel, the austenite grain sizes are made fine in a high carbon steel with a C content of more than 0.7%, thereby securing the drawability. For example, U.S. Pat. No. 5,156,692 discloses the following technique. That is, a deformation is controlled to be undergone at a high temperature, and thus, the grain size of the austenite is controlled to about 5 &mgr;m. In this manner, the drawability is improved by making the inter-lamellar spacing fine and by forming the fine pearlite colonies.
Another example is Japanese Patent Laid-open No. Hei-6-136452. In this method, when carrying out the patenting, AlN is precipitated, thereby inhibiting the growth of the austenite grains. However, if the austenite grains are made fine in this manner, in the case of a medium carbon steel, the volume fraction of the ferrite is increased, so that the drawability is rather aggravated. Accordingly, this method cannot be applied to the medium carbon steel.
As still another example, there is Japanese Patent Laid-open No. Hei-4-325627. In this method, a large amount of Si is added to the steel, and thus the strength and ductility of the steel are improved by the solution hardening. However, if Si is added in a large amount, decarburization is caused during the rolling.
Besides, there are other methods of improving the strength and the ductility by adding alloy elements or by controlling the cooling rate. Typical examples of them are Japanese Patent Laid-open No. Sho-63-4039, Hei-4-346619 and Hei-4-254526.
In the case of Japanese Patent Laid-open No. Sho-63-4039, there is prepared a steel which contains 0.7-0.95% of C, 0.2-0.5% of Si, 0.4-0.7% of Mn, 0.05-0.2% of V, and 0.05-0.5% of Ni. The drawing and patenting are repeated to manufacture a wire of about 0.3 mm.
In the case of Japanese Patent Laid-open No. Hei-4-346619, a carbon steel which contains, in wt %, 0.6-1.1% of C, 0.1-0.2% of Si, and 0.1-2.0% of Mn is subjected to a patenting. Then a drawing is carried out by more than 60%, and then, the steel is maintained at a temperature of 50-200° C. for 5 minutes to 1 hour. Thus the ductility aggravation which is caused by the strain aging during the drawing is compensated, thereby obtaining a superior steel wire.
However, in the above two methods, the ductility of the steel wire cannot be increased, and therefore, there is a problem in increasing the drawing strain without carrying out the patenting.
In Japanese Patent Laid-open No. Hei-4-254526, a steel which contains 0.9-1.3% of C, 0.1-2.0% of Si and 0.1-1.3% of Cr is hot-rolled. Then a rapid cooling is carried out down to a temperature at which the pro-eutectoid cementite is produced. Then a cooling is carried out at a rate of 8° C./sec down to a temperature at which the pearlite transformation is terminated. Or the rapid cooling is carried out down to the pearlite transformation temperature, and then, the steel is isothermally maintained, thereby inhibiting the formation of the pro-eutectoid cementite, and improving the ductility of the drawn wire. In this method, however, the pro-eutectoid cementite is not formed at a carbon content of less than 0.9%, and therefore, the method cannot be applied to this case. Further, after the actual rolling, there is a difficulty in controlling the cooling by dividing the cooling step into two stages.
As described above, in most of the conventional techniques, an intermediate heat treatment called patenting is necessarily carried out during the drawing. That is, the patenting is for controlling the strained structure which has been formed during the drawing. It is a well known fact that the patenting has to be necessarily undergone if the wire is to be drawn to the final wire diameter.
However, if the drawability is ensured without carrying out the patenting, then there are various advantages as follows. That is, the raw stock can be drawn directly to the final product, and the pickling for removing the scales produced as a result of the patenting can be skipped. Further, the lubricant coating for carrying out the drawing can also be skipped. In fact, however, if wire drawing is carried out without the patenting, the ductility of the stock is markedly aggravated due to the work hardening, with the result that breaking may occur during the drawing, and that the delamination may be found after the drawing. The delamination is increased proportionally to the strength of the stock and to the drawing strain. Particularly it is known that if the drawing strain is increased, the delamination is more frequent compared with the case where the strength of the stock is strengthened.
Meanwhile, in steel wires such as bead wire, it is required that the elongation be more than 5%. Therefore conventionally, in order to secure the elongation, a carbon steel of 0.7-0.8%C was subjected to drawing, patenting and drawing, and then, bluing was carried out in a Pb bath. However, the bluing tends to cause lowering of the strength of the steel wire proportionally to the recovery of the elongation. That is, if the bluing is carried out in the general manufacturing method, the elongation is restored, but the tensile strength is lowered by about 20 Kg/mm
2
. Therefore, a steel wire which has a tensile strength of 250 Kg/mm
2
will have a tensile strength of 230 Kg/mm
2
after the bluing. If a strength of 200 Kg/mm
2
is to be obtained in a bead wire, at least a strength of 220 Kg/mm
2
has to be secured. However, in the case of the usual carbon steel, if the drawing strain is 95% or more, the elongation is recovered by not more than 5%. Thus, in order to secure the elongation, if the bluing is carried out at a high temperature, the tensile strength is greatly lowered (so it is known) (materials letter, 1997, p241). In the case of a low carbon steel which has a superior ductility, the restoration of the elongation is not well realized after the drawing (so it is known) (CAMP-ISIJ vol 8, 1995, p1373). Further, in the usual drawing amount, if the carbon content is less than 0.6%, it is difficult to obtain an elongation of more than 5% after the bluing (so it is known) (CAMP-ISIJ, vol. 11, 1998, p347).
Therefore it is proposed as follows. That is, in the case of the wire rods for making the bead wire, adding alloy elements into a high carbon steel or modifying the bluing process is proposed. For example, Japanese Patent Laid-open No. Hei-5-105966 proposes as follows. That is, in a steel which contains 0.9-1.1% of C and Cr and Mn, the patenting conditions are modified to make the fine structure become a bainite so as to obtain a bead wire with a strength of 250 Kg/mm
2
and an elongation of 8%. Japanese Patent Laid-open No. Hei-1-165795 proposes the following technique. That is, the bluing is not carried out after the dr

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