JP2008038199A - Method for recovering ductility of metallic wire rod - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To give a method for recovering a ductility without sacrificing a tensile strength and a bending strength of a metallic wire rod after applying heat treatment. <P>SOLUTION: When the heat treatment is applied to the metallic wire rod having 4000 MPa or higher of the tensile strength in the temperature range of 250-400°C, the holding time in this temperature zone is controlled so that Fe diffusion distance X(m) in the metallic wire rod after applying heat treatment, becomes in the range of 1.00×10<SP>-13</SP><X<1.00×10<SP>-11</SP>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for recovering ductility of a metal wire. Here, the metal wire is a generic name including the metal wire obtained by drawing the metal wire in addition to the metal wire used as a starting material when manufacturing the wire that is a component of the cord in particular. To do.

  Various characteristics are required for the metal wire that is a constituent element of the cord. For example, from the viewpoint of environmental issues in recent years, there is an urgent need to reduce the weight of tires that contribute to promoting the reduction in fuel consumption of automobiles. For this purpose, there is a demand for increasing the strength of the cord that serves as a reinforcing material for the tire and reducing the amount of use.

  As a technique for increasing the strength of the cord, it is effective to increase the strength of the strands constituting the cord itself. In order to increase the strength of the wire, the composition of the metal wire that is the starting material of the wire obtained by wire drawing is adjusted or the device is devised for wire drawing. . As a result, high strength is achieved, but on the other hand, there is a problem that ductility of the metal wire is lowered with the increase in strength.

  Conventionally, as a means for recovering the ductility of a metal wire, it is common to subject the metal wire to a low-temperature and short-time heat treatment, so-called bluing treatment. Ductility recovery is achieved by applying the blueing treatment to the metal wire.

  For example, the breaking elongation of the steel cord is increased by subjecting the steel cord having a tensile strength of less than 3000 MPa to a blueing treatment that is performed at a constant holding time in a temperature range near 400 ° C. (See Patent Document 1 and Patent Document 2)

In addition, the steel wire is subjected to wire drawing, plating, and blueing treatment for several seconds to several tens of seconds in a temperature range of 340 ° C. to 500 ° C. to increase elastic elongation (see Patent Document 3). .
Furthermore, the maximum value of the internal friction of the carbon steel wire can be increased by applying a blueing treatment that adjusts the holding time between 6 seconds and 15 minutes in the temperature range of 250 ° C to 440 ° C. Ductility is improved by setting it in a suitable range in the temperature range from ℃ to 220 ℃. (See Patent Document 4)
JP-A-9-228274 Japanese Unexamined Patent Publication No. 2001-512191 Japanese Unexamined Patent Publication No. 2000-80441 Japanese Patent Laid-Open No. 11-269557

  As described above, conventionally, a technique for recovering the ductility of a high strength metal wire has been generally a blueing treatment.

  However, by applying the blueing treatment to the metal wire, the ductility is restored and the tensile strength of the metal wire is reduced by 15% or more. Moreover, since cementite which is an internal structure | tissue of a metal wire is spheroidized, bending strength will fall. This is because the ductility is restored by the bluing treatment, but at the same time, the lamellar structure (layered structure of ferrite and cementite) extending in the processing direction, for example, by wire drawing is divided, resulting in a decrease in bending strength and further removal of strain. The strength reduction that exceeds the strength reduction associated with.

  Here, the bending strength means that in the hook test shown in FIG. 1, two metal wires 1 are looped and hooked together, and each metal wire 1 is the same at the center between the grips 2 of the tensile tester. The metal wire is fixed so as to be in parallel contact, the curvature of the hooked portion of the metal wire is made constant, and the load when the metal wire is pulled and broken is meant.

  Accordingly, an object of the present invention is to provide a way to recover ductility without sacrificing the tensile strength and bending strength of the metal wire after heat treatment.

  In recovering the ductility of the metal wire, it has been improved by adjusting the conditions of the heat treatment. At the same time, however, since the tensile strength and bending strength of the metal wire after the heat treatment are lowered, it is difficult to improve both in a balanced manner.

  Therefore, the inventors have intensively studied means for recovering ductility without reducing the tensile strength and bending strength of the metal wire after heat treatment. As a result, when the heat treatment is performed on the metal wire, the heat treatment temperature and the holding conditions are maintained. By adjusting the time, it was found that the ductility was restored without sacrificing the tensile strength and bending strength of the metal wire, and the present invention was completed.

That is, the gist configuration of the present invention is as follows.
(1) When a metal wire is heat-treated in a temperature range of 250 ° C. or more and 400 ° C. or less, the retention time in the temperature range is set so that the Fe diffusion distance X (m) in the metal wire after the heat treatment is 1.00 × 10 ^{− 13} <X <1.00 × 10 ^{−11 In} a ductility recovery method for a metal wire, characterized by controlling in a range.

(2) The ductility recovery method for a metal wire according to (1), wherein the metal wire has a tensile strength of 4000 MPa or more.

(3) The ductility recovery method for a metal wire according to (1) or (2), wherein the heat treatment is performed under reduced pressure or in an inert gas.

  According to the present invention, when heat treating a metal wire, the ductility can be recovered without sacrificing the tensile strength and bending strength of the metal wire by adjusting the heat treatment temperature and holding time in a well-balanced manner. It becomes possible to provide a metal wire.

The present invention intends to recover the ductility of the metal wire and the metal wire used for the production of the strand by performing a heat treatment, in particular, by reducing the ductility of the metal wire which has been lowered with increasing strength.
That is, when the metal wire is subjected to heat treatment in a temperature range of 250 ° C. or more and 400 ° C. or less, the holding time in the temperature range is set such that the Fe diffusion distance X (m) in the metal wire after the heat treatment is 1.00 × 10 ⁻¹³ < Control is performed in a range of X <1.00 × 10 ⁻¹¹ .

Here, the reason for setting the temperature of the heat treatment applied to the metal wire in the range of 250 ° C. or more and 400 ° C. or less is as follows. That is, when the temperature is less than 250 ° C., Fe, which is a component of the metal wire material, hardly diffuses, and only strain aging that cures when carbon moves and adheres to the strain appears, and ductility decreases.
On the other hand, when the temperature exceeds 400 ° C., the movement of Fe atoms becomes active, and cementite tends to spheroidize in addition to strain removal, and the Fe diffusion distance X (m) is within the range of 1.00 × 10 ⁻¹³ <X <1.00 × 10 ⁻¹¹ In order to achieve this, the heat treatment holding time must be 1 second or less, and such a short time holding is difficult to control.
More preferably, the temperature range is 250 ° C. or higher and 350 ° C. or lower.

And, in performing the heat treatment in the above temperature range, it is important to control the holding time so that the Fe diffusion distance X (m) after the heat treatment is 1.00 × 10 ⁻¹³ <X <1.00 × 10 ^−11. It is. The Fe diffusion distance X (m) is calculated by the following formula (1).
Record
X = √2 · D · t ---- (1)
However, D: D ₀ · e ^{(-Q / RT)}
t: Retention time (s)
Here, D ₀ : diffusion coefficient 2.80 × 10 ⁻⁴ (m ² / s)
-Q: Activation energy 251 (kJ / mol)
R: Gas constant
T: Temperature (K)

  Here, the significance of controlling the holding time by the Fe diffusion distance X (m) is that strain is introduced as a crystal lattice defect of Fe, and in order to remove the strain, a slight movement (diffusion of Fe atoms) Therefore, if the holding time, which is a heat treatment condition, is controlled by the Fe diffusion distance X (m), the strain can be surely removed.

Therefore, if the Fe diffusion distance X (m) in the metal wire after heat treatment is in the range of 1.00 × 10 ⁻¹³ <X <1.00 × 10 ⁻¹¹ , the decrease in tensile strength after heat treatment is suppressed to 10% or less. It is done. In addition, the bending strength is improved without a large change in elongation.

In other words, with a holding time where the Fe diffusion distance X (m) is 1.00 × 10 ⁻¹³ or less, sufficient strain removal cannot be performed, and because carbon adheres to the strain, strain aging preferentially develops. Cannot be recovered.
Further, in the holding time where the Fe diffusion distance X (m) is 1.00 × 10 ⁻¹¹ or more, strain is removed, while on the other hand, cementite spheroidization, which is a reaction involving diffusion of Fe atoms responsible for strain removal, or Recrystallization will occur and the strength will be greatly reduced.
More preferably, 5.0 × 10 ⁻¹³ <X <5.0 × 10 ⁻¹² is satisfied.

  The heat treatment is preferably applied to a metal wire having a tensile strength of 4000 MPa or more. This is because a metal wire having a pressure of 4000 MPa or more is likely to be significantly reduced in ductility due to delamination or the like, and thus it is effective to restore the ductility by applying the heat treatment of the present invention to such a wire.

  Further, the heat treatment of the metal wire is preferably performed under reduced pressure or in an inert gas. This is because when the metal wire is subjected to heat treatment at 250 to 400 ° C. in the atmosphere, the surface of the metal wire is oxidized, and when the oxidized metal wire is used for reinforcement of a tire or the like, it adheres to rubber. Therefore, it is preferable to carry out under reduced pressure or in an inert gas effective for suppressing oxidation. It is also possible to remove the oxide film of the metal wire, but rather than adding the removal process to the metal wire manufacturing process, heat treatment is performed under reduced pressure or in an inert gas that suppresses oxidation of the surface of the metal wire. It is more efficient to implement.

  A metal wire having a tensile strength of 4500 MPa was subjected to heat treatment under the conditions shown in Table 1. The metal wire after the heat treatment was measured for tensile strength, breaking strain, and Fe diffusion distance X (m). These results are also shown in Table 1.

  In addition, the Fe diffusion distance (m) in Table 1 is obtained based on the above formula (1).

  For the tensile strength of the metal wire after heat treatment, create a stress-strain diagram based on the tensile test in accordance with JIS Z 2241, obtain the maximum stress from the stress-strain diagram, and calculate the value. The tensile strength.

  For the breaking strain after the heat treatment, a stress-strain diagram was created based on a tensile test based on JIS Z 2241, the elongation at break was determined from the stress-strain diagram, and the value was taken as the breaking strain.

  Further, the bending strength characteristic index is obtained by indexing the bending strength measured in the hook test of FIG.

It is explanatory drawing of the hook test which measures bending strength.

Explanation of symbols

1 Metal wire
2 Grip of testing machine

Claims (3)

When heat treatment is performed on a metal wire in a temperature range of 250 ° C. or more and 400 ° C. or less, the holding time in the temperature range is set as follows: Fe diffusion distance X (m) in the metal wire after the heat treatment is 1.00 × 10 ⁻¹³ <X <Method for recovering ductility of metal wire, characterized by being controlled within range of 1.00 × 10 ^-11   The method for recovering ductility of a metal wire according to claim 1, wherein the metal wire has a tensile strength of 4000 MPa or more.   The method for recovering ductility of a metal wire according to claim 1 or 2, wherein the heat treatment is performed under reduced pressure or in an inert gas.

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