JP2675857B2 - Steel cord wire manufacturing method and steel cord - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a steel cord wire constituting a steel cord used as a reinforcing material for a tire, and the steel cord.

[Prior Art] Generally, as a raw material of a wire for steel cord,
0.72% C hypoeutectoid steel or 0.82% C eutectoid steel piano wire is used.

The three main factors that determine the tensile strength of this type of wire are the wire component (mainly C%), heat treatment (patenting), and wire drawing workability.

Here, if the tensile strength is further improved by using 0.72% C hypo-eutectoid steel or 0.82% C eutectoid steel piano wire as the raw material, the strength obtained in terms of heat treatment is determined by the wire material composition. As a result, the wire drawing workability must be greatly increased. However, if the degree of wire drawing is increased too much, the strength is improved, but on the other hand, the toughness of the steel cord is deteriorated, and as a result, the fatigue resistance of the steel cord is deteriorated.

Therefore, the strength of this type of wire that can withstand conventional practical use is 350 kgf / m in the wire diameter range of 0.15 to 0.38 mm.
The limit was m ² .

Therefore, the strength of the wire itself is high as a raw material for the wire.
It is conceivable to use a hypereutectoid steel piano wire (JIS G3502) of 0.85 to 0.95% C. When such raw materials are used, the strength of the wire itself is higher than that of 0.72% C hypoeutectoid steel and 0.82% C eutectoid steel piano wire, and high strength can be obtained by heat treatment. It is possible to obtain a strength of 350 kgf / mm ² or more without increasing the workability extremely.

[Problems to be Solved by the Invention] However, when a wire for steel cord is to be obtained by using a hypereutectoid steel piano wire of 0.85 to 0.95% C as a raw material as described above, according to the conventional heat treatment method, the first eutectoid during heat treatment There is a problem that subsequent wire drawing becomes difficult because cementite is generated and coarsens.

The present invention has been made by paying attention to such a point, and an object thereof is to manufacture a wire for a steel cord which suppresses the formation of pro-eutectoid cementite during heat treatment and facilitates wire drawing after heat treatment. To provide a method and a steel cord.

[Means for Solving the Problems] In the present invention, a hypereutectoid steel piano wire rod of 0.85 to 0.95% C is drawn, and during the final heat treatment of this wire rod, the wire rod is heated to undergo austenite transformation. The steel cord wire is obtained by setting the time from the heating of the wire in the process of cooling to the pearlite transformation temperature to the passage of the A ₁ transformation point to 0.8 seconds or less.

[Operation] In such a method, generation of pro-eutectoid cementite during heat treatment is suppressed, and a wire having an optimum pearlite structure is obtained. Therefore, this wire can be easily drawn under normal drawing conditions to finally obtain a steel cord wire having a tensile strength of about 355 to 365 kgf / mm ² . A plurality of such wires can be twisted together to form a steel cord having excellent fatigue resistance.

The reason why the fatigue resistance is excellent can be explained as follows. First, in the present invention, since it is possible to suppress the formation of proeutectoid cementite during heat treatment,
A hypereutectoid steel piano wire, which has a higher carbon concentration than the conventional wire, can be used. The high carbon concentration means that the amount of cementite (Fe ₃ C) increases, and the width of the ferrite layer in the pearlite structure becomes relatively narrow.
As in the schematic diagram shown in the figure, the free slip stroke becomes shorter.
Therefore, dislocations generated in the ferrite layer in the pearlite structure are accumulated at the ferrite-cementite interface to generate stress concentration as shown in the schematic diagram in Fig. 2, and the fatigue fracture that develops due to fracture of the cementite layer is small. Becomes
Fatigue resistance is improved. This is also clear from the ferrite layer width (L ₀ ) and the stress (σ _c ) applied to the cementite layer, and the following relational expression.

σ _c = {4 / (μ · b)} L ₀ · σ ² where μ: rigidity ratio b: Burger spectrum σ: deformation stress.

[Examples] Table 1 below shows the chemical components of the wire material, No. 1 is related to the present invention, and No. 2 and No. 3 are comparative products.

These three types of wire rods were drawn by a normal wire drawing means from a wire diameter of 5.5 mm to a wire diameter of 1.20 mm, 1.45 mm, 1.50 mm, 1.55 mm. Then, these wires are subjected to final heat treatment at a heating temperature of 950 ° C. and a cooling lead bath temperature of 550 ° C., and the time from the heating treatment of each wire to entering the cooling lead bath at this time,
In other words, as shown in Table 2, the A ₁ transformation point passage time of the wire is 0.8
The processing was performed separately for seconds and 1.3 seconds. After this heat treatment, the wire rod is subjected to plus plating, and further drawn into a wire diameter of 0.25 mm to make a wire for steel cord. Such a wire of the same kind is twisted in a (1 x 5) structure to make a steel cord. And The properties of the wire and steel cord thus obtained are shown in Table 3.

As is clear from Table 3, No. 1 obtained by applying the present invention
It can be seen that -a and No. 1-b show better tensile strength and fatigue resistance than the others. N
For o.1-c and No.1-d, the wire material is No.1-
a and No. 1-b, but the A ₁ transformation point transit time during heat treatment exceeds the range specified in the present invention.
Since the characteristics are deteriorated because it is set to 1.3 seconds, the wire diameter of 1.50 mm at the time of final heat treatment causes No.1-d wire breakage during wire drawing after heat treatment, resulting in a 0.25 mm wire. Processing to the diameter was impossible. In the case of No. 1-c with the final heat treated wire diameter of 1.45 mm, wire drawing up to 0.25 mm was possible, but fatigue resistance deteriorated due to the twisted state as a steel cord. ing.

No. 2 which uses 0.82% C eutectoid steel piano wire as the raw material
-A, No.2-b, No.2-c, the wire drawing degree is N
Even if it is made larger than that of o.1-b, the tensile strength cannot be made larger than that of No.1-b, and on the contrary, deterioration of toughness occurs due to high wire drawing workability and fatigue resistance decreases. Is appearing. No.2-d ones were set the same as A ₁ transformation point passing time as the No.1-d to 1.3 seconds, the saw lowering properties, such as occurs in those No.1-d I can't. It is proved that No. 2-is the eutectoid steel, so that there is no formation of pro-eutectoid cementite and that the obtained properties are the same even if the wire is drawn with the same workability.

In addition, 0.72% C hypoeutectoid steel piano wire is used as the raw material.
Items No. 3-a and No. 3b are listed for reference. No.3-
The item a is a conventional general steel cord manufacturing condition, which was used as a base for comparison of 3-roll fatigue properties. No.3-b
No. 1-a and No. 2-a are compared, and the difference in characteristics when the carbon concentration of the wire is changed at the same wire drawing degree is examined.

As described above, according to the present invention, 0.85 to 0.95% C
Over eutectoid steel piano wire and wire drawing, when the final heat treatment of the wire, A ₁ transformation point after heating the wire rod in the course of cooling to the wire to heat to pearlite transformation temperature of mixture was allowed to austenite transformation Since the heat treatment was carried out by setting the time until it passed through 0.8 seconds or less, it was possible to suppress the formation of pro-eutectoid cementite during the heat treatment to obtain a wire with an optimum pearlite structure and draw the wire after heat treatment. It is easy to carry out, and therefore a plurality of such wires can be twisted together to form a steel cord having excellent fatigue resistance.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing the relationship between the C concentration and the free slip stroke, and FIG. 2 is an explanatory view schematically showing the fatigue fracture process.

Claims (2)

(57) [Claims] 1. A process of drawing a 0.85-0.95% C hypereutectoid steel piano wire rod, and heating the wire rod during the final heat treatment of the wire rod to perform austenite transformation and then cool it to the pearlite transformation temperature. Heat treatment to suppress the formation of pro-eutectoid cementite by setting the time from the heating of the wire rod until it passes the A ₁ transformation point to 0.8 seconds or less, and after this heat treatment, the wire rod is stretched at a workability of 97% or more. A method for manufacturing a wire for a steel cord, which is characterized in that the wire is processed. 2. A process of drawing a 0.85-0.95% C hypereutectoid steel piano wire rod, and heating the wire rod during the final heat treatment of the wire rod to transform it to a pearlite transformation temperature after austenite transformation. Heat treatment to suppress the formation of pro-eutectoid cementite by setting the time from the heating of the wire rod until it passes the A ₁ transformation point to 0.8 seconds or less, and after this heat treatment, the wire rod is stretched at a workability of 97% or more. A steel cord made by wire processing and then twisting together the multiple wires obtained in this way.