JPH07268546A - High carbon steel wire rod having two-layer structure and its production - Google Patents

Abstract

PURPOSE:To produce a high carbon steel wire rod high in tensile strength, in which delamination is hard to generate and capable of high degree wire drawing and to provide a method for producing the same. CONSTITUTION:This high carbon steel wire rod is the one contg. 0.7 to 1.2wt.% C and having a two layer structure in which a bainitic structure or a structure in which cementite is spheroidized is formed from the surface to a depth of 5 to 20%of the radius of the cross section and a pearlitic or ferrite pearlitic structure is formed at the part inner than the same. This wire rod can be obtd. by heating a high carbon steel wire stock contg. 0.7 to 1.2wt.%. C to 850 to 1100 deg.C and thereafter bringing it into contact with a coold roll to rapidly cool the surface layer part and to form the structure of the surface layer part into a bainitic one or forming the structure of the surface layer part into pearlite, bainite or martensite or a mixted one thereof and executing rolling to spheroidize cementite in the surface layer part structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon steel wire rod for wire drawing, which is excellent in delamination resistance and has a high tensile strength after wire drawing, and a method for producing the same.

[0002]

2. Description of the Related Art High carbon steel wire rods are usually drawn.
It will be the final product such as reinforced steel wire for power transmission cable, steel cord wire, and steel bead wire. This wire rod for wire drawing has conventionally been patented to have a uniform cross-section before wire drawing. As the patenting method, air patenting in which the structure of the wire is once austenized and then air-cooled or air-cooled, or lead patenting in which the wire is immersed in lead is often used.

"Iron and Steel" Vol.79 (1993), No.9, p.89-95
As shown in (Reference 1), among the structures obtained by patenting, it is excellent in ductility during wire drawing and delamination resistance (durability of vertical cracking in a twist test of a wire drawn material) and It is said that the structure having only fine pearlite has the highest tensile strength afterwards. Nevertheless, as shown in Fig. 6 on page 92 of the above-mentioned document 1, wire drawing strain
Delamination begins to occur when (ε) exceeds 2.53. The fact that delamination easily occurs means that
This means that it is impossible to perform a high degree of wire drawing, which means that the wire drawability is poor.

Therefore, as shown in CAMP-ISJI, Vol.6 (1993) -1684 (Reference 2) and Japanese Patent Laid-Open No. 5-117762, a bainite uniform structure is used to delaminate even in a high wire drawing area. A high-carbon steel wire rod that is less likely to generate and a method for manufacturing the same have been proposed. However, as shown in Fig. 2 of Reference 2, the bainite wire rod has a drawback that the final tensile strength after wire drawing is low because the work hardening rate is low, although the occurrence of delamination is suppressed. is there.

[0005]

As described above, a wire having a uniform fine pearlite structure has a high tensile strength after drawing, but delamination is more likely to occur than a wire having a uniform bainite structure. On the other hand, a wire having a bainite structure has less delamination but a low final tensile strength after wire drawing. That is, at present, no wire rod has been obtained in which delamination is unlikely to occur and which has a high tensile strength after wire drawing.

It is an object of the present invention to provide a high carbon steel wire rod which has a high tensile strength after wire drawing, is less likely to cause delamination, and is capable of high-level wire drawing, and a method for producing the same. It is a thing.

[0007]

Means for Solving the Problems The inventors of the present invention have variously changed the metallographic structure of a high carbon steel wire rod, and have produced a wire rod having a different surface layer portion and central portion structure, which are drawn and drawn. A detailed study was conducted on the tensile strength and the occurrence of delamination. As a result, it was found that the drawability and the tensile strength after drawing are mainly influenced by the structure of the central part, and the easiness of delamination is greatly affected by the structure of the surface part.

Specifically, if the center part has a uniform pearlite structure, the wire drawability is
It is possible to obtain a wire rod which is excellent in (drawability as referred to here is the ability to draw wires finely without breaking) and which has high tensile strength after drawing. However, if the pearlite structure extends to the surface layer, delamination easily occurs. That is, vertical cracks are likely to occur in the twisting test after wire drawing.

If the surface layer portion has a uniform bainite structure, delamination is less likely to occur.

However, if the entire cross section has a bainite structure, the tensile strength after wire drawing is low.

Similarly, if the surface layer has a soft structure in which cementite is spherical, delamination is less likely to occur. However, if the entire cross section has a spheroidized cementite structure, the wire drawability is poor and the tensile strength after wire drawing is low.

The present invention was made on the basis of the above findings, and is a wire rod having a two-layer structure having the structures of the surface layer portion and the central portion different from each other in the following (1) and (3), and (2) and (4) The manufacturing method of those is summarized.

(1) It contains 0.7 to 1.2% by weight of C, a bainite structure is present from the surface to a depth of 5 to 20% of the cross-sectional radius, and the structure inside thereof is a pearlite or ferrite-pearlite structure. A high carbon steel wire rod having a layered structure.

(2) A high carbon steel wire rod containing 0.7 to 1.2% by weight of C is heated to 850 to 1100 ° C. and then brought into contact with a cooling roll to rapidly cool the surface layer portion, and the surface layer portion has a bainite structure. The method for producing a high carbon steel wire rod according to the above (1), characterized in that

(3) Cementite is spheroidized at a depth of 5 to 20% of the cross-sectional radius from the surface containing 0.7 to 1.2% by weight of C, and the internal structure is pearlite or ferrite pearlite. A high carbon steel wire rod having a two-layer structure structure which is a structure.

(4) After heating a high carbon steel wire rod containing 0.7 to 1.2% by weight of C to 850 to 1100 ° C., it is brought into contact with a cooling roll to rapidly cool the surface layer, and the surface layer is pearlite, bainite, Martensite, or a mixed structure of these, with a total processing rate of 15 at the same time as or immediately after the above quenching
% Or more to make the cementite in the surface layer structure spherical, and the method for producing a high carbon steel wire rod according to (3) above.

[0017]

[Operation] First, the wire rods (1) and (3) will be described. These wire rods have a bainite or cementite spheroidized structure in the surface layer and a pearlite or ferrite-pearlite structure in the center.

The content of C (carbon) in the above wire is 0.7-1.
The reason for setting it to 2% (hereinafter,% with respect to the content of components means “% by weight”) is as follows.

When C is less than 0.7%, the tensile strength after patenting (initial strength before wire drawing) is low, and therefore the tensile strength after wire drawing is low, so that it does not meet the purpose of use as a high carbon steel wire. . On the other hand, when C exceeds 1.2%, pro-eutectoid cementite is generated at the crystal grain boundaries, and wire drawability is greatly deteriorated. Therefore, the C content is set to 0.7 to 1.2%. A more desirable range of the C content is 0.7 to 1.0%.

The components other than C and the contents thereof may be selected within the range specified by ordinary high carbon steel wires. For example,
Si: 0.15 to 0.35%, Mn: 0.3 to 0.9%, Cr: 0.4% or less (0
), Ni: 0.3% or less (may be 0), Cu: 0.3% or less
(It may be 0), P: 0.04% or less as impurities, and S: 0.04% or less are standard compositions.

Next, the structure of the surface layer will be described. Here, the surface layer portion is from the surface of the wire to a depth of 5 to 20% of the cross-sectional radius. For example, in the case of a wire with a radius of 10 mm (diameter of 20 mm), the surface layer extends from the surface to a depth of 0.5 to 2 mm, and this portion must have a spheroidized structure of bainite or cementite. If the surface layer portion of such a structure is less than 5% from the surface, the delamination suppressing effect is small, and if it exceeds 20%, the wire drawability deteriorates sharply, and the tensile strength after wire drawing also becomes low.

The structure of the part (central part) excluding the surface layer is a pearlite or ferrite-pearlite structure. It becomes pearlite in eutectoid or hypereutectoid steel, and ferrite pearlite in hypoeutectoid steel. If the center part has a structure other than these, the wire drawability is deteriorated.

Among the wire rods having the above-mentioned two-layer structure, the wire rod whose surface layer portion is bainite can be manufactured by the method (2). In this method, high carbon steel wire rod
The reason for heating to 1100 ° C is that if the temperature is less than 850 ° C, the material structure does not completely become austenite and the carbide remains, so that the pearlite (or ferrite / pearlite) structure is not uniform in the center in the next cooling step. This is because the surface layer portion does not have a bainite structure and a desired two-layer structure cannot be obtained. On the other hand, when the heating temperature exceeds 1100 ° C., the crystal grains are abruptly coarsened and the wire drawability is significantly reduced. It should be noted that the heating time may be a time sufficient for the entire cross section of the wire to have a uniform temperature and to be completely austenitized. For example, when induction heating is performed while the steel material is running, short-time heating for 5 seconds to 1 minute is sufficient.

The wire rod heated as described above is brought into contact with a cooling roll to be cooled. In this way, cooling is performed by contacting with the chill roll by rapidly cooling only the surface layer and rapidly
This is to obtain the above-mentioned two-layer structure by cooling below the r ₁ transformation point. The rapidly cooled surface layer portion has a bainite structure, and the slowly cooled center portion has a pearlite (or ferrite / pearlite) structure.

It is difficult to rapidly cool only the surface layer and slowly cool the central part by a cooling method which is usually performed by passing a heated wire through an air-cooled zone or a water-cooled zone or a method of immersing in a lead bath. . Even if the bainite structure can be formed in the extremely thin area of the surface layer, the thickness of the layer cannot be controlled. However, since the heat transfer coefficient is large in the cooling method using roll contact, only the surface layer portion can be uniformly and instantaneously rapidly cooled, and a target structure can be easily obtained.

In order to increase the cooling rate of the surface layer portion of the wire rod, it is desirable to cool the roll from outside or inside by water cooling or the like. Further, a method of continuously cooling with a plurality of rolls is recommended. The thickness of the surface layer structure can be adjusted by the degree of roll cooling, the number of rolls in contact, and the wire feed rate.

Next, the method for producing the wire according to the above (3), in which the surface layer has a "structure in which cementite is spheroidized" will be described. The chemical composition of the wire and its heating temperature may be the same as described above. Cooling after heating is also performed by a rapid cooling method by contact with a roll. Conditions for this quenching (degree of roll cooling,
The surface layer portion becomes pearlite, bainite or martensite, or a mixed structure of these, depending on the number of rolls to be contacted, the feed rate of the wire, etc.

As described above, the surface layer portion is rapidly cooled to ₁ point or less of Ar to transform only the surface layer portion into pearlite, bainite or martensite, or a mixed structure thereof, and then immediately rolled to obtain these structures. Sphericalize cementite inside. This rolling may be performed simultaneously with roll cooling. That is, the cooling roll has a role of a rolling roll, and rapid cooling and rolling are performed at the same time. Further, rolling may be carried out immediately after the roll is rapidly cooled in a line where cooling rolls and rolling rolls are arranged. As a rough guide, it is recommended to perform rolling before the temperature of the wire center falls below Ar ₁ point + (100 to 200 ° C). The recuperation from the central part of the wire promotes the spheroidization of cementite in the surface layer subjected to the processing.

In rolling, the total processing rate (area reduction rate) is 15%
Do as above. At less than 15% rolling, cementite does not spheroidize sufficiently. The quenching and rolling may be continuously performed with a plurality of rolls. Even if the cementite in the surface layer is not completely spheroidized (for example, even if the spheroidization rate is about 60%), the delamination suppressing effect is sufficient.

[0030]

Example 1 A wire having a two-layer structure in which the surface layer portion was bainite and the central portion was pearlite was manufactured as follows. First,
Cold draw a 5.5 mmφ wire rod with the chemical composition shown in Table 1
A wire having a diameter of mmφ was prepared, and the heat treatment shown in Table 2 was performed while feeding the wire at a speed of 3.5 m / s.

Roll cooling is 200 mm with a hole type of 3 mmφ
The thickness of the bainite layer in the surface layer portion was adjusted by changing the number of rolls in contact with the wire rod by using a line in which 12 high-speed steel rolls of mm were continuously arranged. For comparison, a method of cooling by immersion in a lead bath was also carried out. The cross-sectional structure of the wire thus obtained was observed with an optical microscope, and the average depth of the bainite layer was determined from the measured values in 12 fields of view. The values are shown in Table 2.

The wire was washed with sulfuric acid and subjected to a zinc phosphate coating lubrication treatment, and then drawn to 0.4 mmφ according to the pass schedule shown in Table 3 using a wet ultrafine wire drawing machine. Then, a sample of each wire drawing process is taken, a twist test is performed at a span of 100 times the wire diameter, and wire drawing strain ln (Ao / An) (where Ao: before wire drawing) at which delamination begins to occur (3 mmφ) cross-sectional area, An: cross-sectional area after wire drawing] and the tensile strength of the final wire-drawn material (0.4 mmφ) were investigated. These measured values are also shown in Table 2 and shown in FIG.

As is clear from FIG. 1, when the depth of the bainite layer from the surface is less than 5% of the wire radius, the strain at the time of delamination is small. That is, delamination is likely to occur. On the other hand, when the depth of the bainite layer exceeds 20% of the wire radius, the tensile strength after wire drawing sharply decreases. Further, as can be seen from Test Nos. 10 and 11 in Table 2, if the heating temperature before cooling the roll is too low or too high, the wire is broken during wire drawing, and a wire having good wire drawability cannot be obtained.

In Examples (Nos. 3 to 5) of the present invention in which the heating temperature and the surface depth of the bainite layer were appropriate, excellent wire drawability and high strength after wire drawing were obtained.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

Example 2 A wire having a two-layer structure in which the surface layer has a spheroidized cementite structure and the center has a pearlite structure was manufactured as follows.

A wire rod having a diameter of 5.5 mm was prepared by simultaneously rolling the wire rod having a chemical composition of 8 mmφ shown in Table 1 by a four-stand rolling mill while changing the feed rate while cooling. Table 4 shows specific conditions of the manufacturing method thereof and the spheroidized structure depth from the surface in the cross-sectional structure measured in the same manner as in Example 1.

In Comparative Example 1 shown in Table 4, lead cross-section (lead bath temperature: 600 ° C.) was used to form a pearlite structure on the entire cross section, and Comparative Example 10 was spheroidized to spheroidize the whole cementite. The organization was Test on Figure 2
The structures of the surface layer and the center of the wire of No. 5 are shown.

The above 5.5 mmφ wire was washed with sulfuric acid and subjected to a zinc phosphate coating lubrication treatment, and then drawn to 0.65 mmφ according to the pass schedule shown in Table 5 using a wet ultrafine wire drawing machine.

Then, a sample is taken in each wire drawing step, a twisting test is conducted in the same manner as in Example 1, and a wire drawing strain ln (Ao / An) at which delamination begins to occur (where Ao: before wire drawing) (5.5 mmφ) cross-sectional area, An: Cross-sectional area after wire drawing)
And the tensile strength of the final drawn wire (0.65 mmφ) was checked.

The results are also shown in Table 4.

As is apparent from Table 4, the depth of the spheroidized tissue layer from the surface is within the range defined by the present invention (section radius of 5 to 5).
If it is shallower than 20%, the strain when delamination occurs is small. That is, delamination is likely to occur
(Test No. 1 to 3). On the other hand, if the depth of the spheroidized tissue layer from the surface is deeper than the above range, wire drawability becomes poor, and 0.
Is wire drawing up to 65 mmφ impossible (Test Nos. 9 and 10),
Even if possible, the tensile strength after wire drawing is drastically reduced.
(Test No. 8). Further, as can be seen from No. 11 and No. 12 in Table 4, even if the heating temperature before cooling and rolling is not appropriate, the wire drawability is poor and the wire is broken during wire drawing.

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

The high carbon steel wire rod having a two-layer structure of the present invention has a high tensile strength after wire drawing, is less likely to cause delamination, and is excellent in wire drawability. This wire is
It can be easily manufactured by the method of the present invention in which the surface layer portion is rapidly cooled by roll cooling.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a depth of a bainite layer in a surface layer portion of a wire having a two-layer structure, a tensile strength of the wire after drawing, and a tendency of delamination to occur.

FIG. 2 is a surface layer portion (a) of a wire having a two-layer structure of the present invention.
FIG. 4 is a diagram showing an example of a metallographic structure of the central part (b).

Claims (4)

[Claims] 1. A two-layer structure containing 0.7 to 1.2% by weight of C, a bainite structure extending from the surface to a depth of 5 to 20% of a cross-sectional radius, and a structure inside the bainite structure being a pearlite or ferrite-pearlite structure. A high-carbon steel wire rod having a structural structure. 2. A high-carbon steel wire rod containing 0.7 to 1.2% by weight of C is heated to 850 to 1100 ° C. and then brought into contact with a cooling roll to rapidly cool the surface layer portion to form a bainite structure. The method for manufacturing a high carbon steel wire rod according to claim 1, wherein 3. A structure containing 0.7 to 1.2% by weight of C, in which cementite is spheroidized from the surface to a depth of 5 to 20% of the cross-sectional radius, and the internal structure is a pearlite or ferrite-pearlite structure. A high carbon steel wire rod characterized by having a two-layer structure structure of 4. A high carbon steel wire rod containing 0.7 to 1.2% by weight of C is heated to 850 to 1100 ° C. and then contacted with a cooling roll to rapidly cool the surface layer, and the surface layer is pearlite, bainite or martens. The cementite in the surface layer structure is spheroidized by rolling at a total working rate of 15% or more at the same time as or immediately after the quenching, as a site or a mixed structure thereof. Carbon steel wire rod manufacturing method.