JP3384204B2 - High strength and high toughness ductile steel wire and method for producing - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is represented by high carbon hot-dip galvanized steel wire represented by PC steel wire, galvanized steel twisted wire, spring steel wire, suspension bridge cable, etc., steel cord wire and the like. TECHNICAL FIELD The present invention relates to a high carbon steel wire which is subjected to a two-phase plating diffusion heat treatment, that is, a steel wire which is subjected to a temperature rising treatment after cold working, and a method for producing such a steel wire, and particularly to a method for heating a temperature during plating. The present invention relates to a high-strength, high-toughness / ductile steel wire which is excellent in toughness and ductility while suppressing the strength reduction at the time of exhibiting excellent strength, and a manufacturing method thereof. The steel wire targeted by the present invention is not limited to the high carbon steel wire as described above, but includes the medium carbon steel wire having a C content of about 0.6% as shown in the following examples. In the following, a high carbon steel wire will be taken up as a typical example for the explanation.

[0002]

2. Description of the Related Art In producing a PC steel wire and a cable for suspension bridges, which are required to have corrosion resistance, a high carbon steel wire is subjected to patenting treatment, wire drawing, and then hot dip galvanizing or the like. Is common. In such a manufacturing process, although the strength of the steel wire is enhanced during wire drawing, there is a problem that the strength is reduced during the temperature rising process during hot dipping. Further, as the strength is increased by drawing, the strength decrease during the plating treatment tends to increase, and as a result, it is difficult to increase the strength of the plated steel wire.

It has been known that adding Si in a high amount is effective as a means for increasing the strength of a high carbon steel wire which is subjected to a temperature rising treatment such as hot dipping as described above.
That is, the addition of Si has the effect of increasing the strength of the steel wire after patenting treatment and also the strength of the steel wire after drawing, and the effect of improving the hardenability of the steel wire and suppressing the precipitation of pro-eutectoid cementite. There is also. Moreover, Si is said to be a very effective element for suppressing the strength reduction during the plating treatment and for enhancing the strength of the hot-dip galvanized steel wire. From this point of view, many techniques have been proposed so far for adding a high amount of Si to a steel wire to be hot-dipped. For example, Japanese Patent Application Laid-Open No. 4-246125 discloses that a steel wire to which Si is added up to 1.3% is hot-dipped, and then straightened and blued. In addition, JP-A-4-
No. 325627 discloses limiting the amount of Si added according to the amount of wire drawing. Furthermore, JP-A-6-33
No. 855 discloses controlling the amount of Si added according to the strength and diameter of the steel wire.

On the other hand, in the hot-dip steel wire as described above, the maximum strength of vertical crack generation during the twisting test, which is one of the important characteristics for evaluating the toughness, depends on the wire diameter of the steel wire. It is known that as the wire diameter increases, the critical strength for vertical cracking decreases. From this point of view, for example, Japanese Patent Laid-Open No. 3-249129 proposes a technique for mechanically straightening and suppressing the occurrence of vertical cracks.

That is, most of the techniques proposed so far for achieving high strength and high toughness of steel wire optimize the chemical composition and manufacturing conditions of the high carbon steel wire. It is also important that the steel wire as described above has excellent ductility.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and exhibits excellent strength by suppressing a decrease in strength during a temperature rising process such as plating. At the same time, it is intended to realize a high-strength, high-toughness and ductile steel wire that is excellent in ductility and also exhibits high toughness and does not cause vertical cracks during twisting. It's about achieving it from a completely new perspective.

[0007]

Means for Solving the Problems The present invention which has been able to solve the above-mentioned problems is a steel wire mainly composed of one or more structures selected from the group consisting of fine pearlite, pseudo-pearlite and bainite. It is a high-strength, high-toughness / ductile steel wire having the gist that the average grain size of carbides in the structure is 10 to 50 nm.

In the above high strength, high toughness and ductile steel wire, in the case where the structure is mainly composed of fine pearlite, the above problems are caused if the average grain size of the cementite crystal grains in the lamellar cementite is 10 to 50 nm. Solvable.

A steel wire mainly composed of one or more structures selected from the group consisting of fine pearlite, pseudo-pearlite and bainite, wherein the average grain size D (nm) of the carbides in the structure and the wire diameter R (mm). The above problem can also be solved by satisfying the relationship of the following expression (1). 2.14 × lnR + 6.56 ≦ D ≦ 2.14 × lnR + 46.6 (1) In this high-strength, high-toughness / ductile steel wire, the average particle diameter D (nm) of the carbide in the structure and the wire diameter R It is preferable that (mm) satisfy the relationship of the following formula (2). 2.14 × lnR + 6.56 ≦ D ≦ 2.14 × lnR + 26.6 (2)

When the structure of this high-strength, high-toughness / ductile steel wire is mainly composed of fine pearlite,
If the average particle diameter D of the cementite crystal grains in the lamellar cementite satisfies the above formula (1) or formula (2), the above problems can be solved.

In producing any one of the above high strength, high toughness and ductile steel wire, the true strain ε in wire drawing is 1.0 to
The temperature may be set to 5.0, and the final soaking temperature T _B may be set to 700 ° C. or lower to operate. In this manufacturing method, the final soaking temperature T _B is preferably 300 to 500 ° C.

Further, as a more specific embodiment of the above-mentioned manufacturing method of the present invention, operation is performed so that the average particle diameter D (nm) satisfies the following expression (3). D = -108.7-12.9 × [Si] + 16.4 × ε + 0.320 × T B -17.6 × logH R ... (3) where, [Si]: Si content in the steel wire ( wt%) H _R: average heating rate from T _B -100 ° C. until between T _B -20 ℃ (℃ / sec)

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted extensive research from a crystallographic standpoint at the nanometer (nm) level, which has never been studied for steels having a fine pearlite structure. As a result, the lamella cementite morphology was successfully made into nanometer (nm) level cementite crystals (hereinafter referred to as “nanocrystals”) by appropriately controlling the cold working conditions and the tempering conditions. Then, as shown in FIG. 1 (however, the wire diameter is 5 mm), between the local elongation performance expressed by [tensile strength (TS) × drawing] and the average grain size of cementite in a nanocrystalline state. Indicates that a very high correlation is observed regardless of the chemical composition of the steel wire, and that by appropriately adjusting the average grain size of the cementite, it exhibits high toughness and vertical cracking does not occur, etc. And completed the present invention. That is, in order to optimize the high strength / high ductility balance of a steel wire composed of a fine pearlite structure and achieve high toughness, it is sufficient to control the average grain size of the nanocrystalline state cementite in the lamellar cementite to an appropriate range. As a result, a high carbon steel wire rod having higher strength, higher toughness and ductility was realized.

The above-mentioned lamellar cementite is scientifically considered to be a single crystal, and it is not known until now that it exhibits a nanocrystalline state. The present invention has been made under the novel idea of controlling the morphology of lamellar cementite in the nanocrystalline state, and is made from a completely different viewpoint from the existing techniques up to now.

[0015] Although some studies have been conducted so far regarding the crystal structure of the steel wire, none of them is for controlling the morphology of lamellar cementite in the nanocrystalline state.
For example, “Journal of the Japan Institute of Metals” (Vol. 37, 1973,
(P. 875), the substructure of coarse spheroidized cementite was observed, and the substructure and ductility of coarse cementite were investigated, but this study is not a study on the nanocrystalline state of lamellar cementite. Also, "Journal of the Japan Institute of Metals" (Vol. 40, 1976, 874)
(Page) describes a study on the recovery process of coarse cementite, which captures the recovery of dislocations during heat treatment. Furthermore, "ISIJ" (Vol.17, P
144,1977, A. Inoue AND T. Masumoto Trans.), The effects of alloying elements on the morphological behavior of coarse cementite have been investigated. The impact of
Again, this is not a study on control of nanocrystalline state morphology.
In addition, in the "Journal of the Japan Institute of Metals" (Volume 7, The 13th Annual Scientific Award Commemorative Lecture, 1968, pp. 363-371), spheroidization of lamellar cementite was promoted by cold working from the examination by the extraction residue method. It has been reported that the lamella cementite after spheroidization has a fine and uniform morphology. However, even in this study, there is no description that cementite exhibits a nanocrystalline state, let alone a quantitative description of the relationship between nanocrystals and mechanical properties.

The reason why the form of the carbides in the steel wire of the present invention is limited will be described in more detail centering on a steel wire having a wire diameter of 5 mm. The present inventors first observed the particle size of lamellar cementite. At this time, lamellar cementite was extracted by the extraction residue method and observed by TEM. Further, the magnification is set to 150,000 times, and the particle size of 20 photographs is measured according to the following formula (1) or (2),
The average value was used for evaluation.

FIG. 2 and FIG. 3 show the bluing treatment temperatures of 425 ° C. and 4 ° C., respectively, for steel type A shown in the examples below.
It is a drawing-substitute micrograph showing the metal structure at 75 ° C. As shown in FIGS. 2 and 3, when the grain boundaries of the nanocrystals can be clearly observed in the whole lamella cementite, the morphology of the grain boundaries of the cementite crystals existing in the lamella cementite per unit area is as follows. The average particle size of cementite was determined according to the equation (4). Average particle size of cementite = (lamella cementite area in observed range / number of nano-cementite particles) ^0.5 x 1.13 (4)

On the other hand, when it is difficult to confirm the grain boundaries of the nanocrystals in the photograph as a whole as shown in FIG. 4 (a steel micrograph of the steel type A showing a metallographic structure at a bluing temperature of 300 ° C.), The average particle size of cementite was obtained from the total area and number of particles having clear crystal grain boundaries according to the following equation (5). Average particle size of cementite = [(observed area of nanolamella cementite area) ^0.5 / number of nanocementite particles] x 1.13 (5)

FIG. 5 shows the average particle size of cementite and (TS
It is a graph showing the relationship with (elongation), but the high strength / high ductility balance represented by (TS × elongation) decreases sharply after the average particle size of cementite exceeds 50 nm. Recognize. Therefore, in the present invention, the upper limit of the average particle size of cementite is defined as 50 nm.

By the way, in the long cable for main cable used up to now, the tensile strength: 160 kgf / mm ² or more, with respect to the respective characteristics of strength and twist value,
And twist value: There is a requirement of 14 times or more. That is,
High strength / high twist value characteristic index (TS x twist value)
Is required to be 2240 (160 × 14) kgf / mm ² or more. The relationship between the average particle size of cementite and (TS × twist value) is shown in FIG. 6, and it is understood that the average particle size of cementite should be 50 nm or less in order to satisfy the above-mentioned required characteristics. From this point of view, in the present invention, the upper limit of the average particle size of cementite is set to 50 nm.

As described above, in the present invention, the upper limit of the average particle size of cementite is defined as 50 nm, but the upper limit of the average particle size is more preferably 30 nm or less. The reason for this is as follows. Tensile strength is 20 as a required property for the wire for the next generation long bridge main cable.
0kgf / mm ² class, 240kgf / m if necessary
It is said that a twist value of 14 times or more and an elongation of 4% or more are required in the m ² class. That is, (TS x twist value) is 33
It is necessary that 60 (240 × 14) kgf / mm ² or more and (TS × elongation) be 9.6 (240 × 0.04) kgf / mm ² or more. In order to satisfy such required characteristics, from the above-mentioned FIG. 5 and FIG.
It can be seen that it may be set to nm or less. Therefore, in the present invention, the preferable upper limit of the average particle size of cementite is set to 30 nm.

Further, according to FIG. 6, when the average particle size of cementite becomes smaller than 10 nm, (TS × twist value) starts to rapidly decrease, and sometimes vertical cracks occur (marked with ●, ■ mark). This tendency is the same in FIGS. 1 and 5. Therefore, in the present invention, the lower limit of the average particle size of cementite is specified to be 10 nm.

As described above, in a 5 mmφ steel wire having a fine pearlite structure, high strength, high toughness and ductility were achieved by defining the average particle size of cementite in lamellar cementite to be 10 to 50 nm. However, according to the studies conducted by the present inventors, such a tendency is not limited to a steel wire in which the structure of the steel wire is composed of a fine pearlite structure, and a steel wire mainly containing pseudo-pearlite or bainite or a steel wire containing the structure as a structure. The average grain size of the carbide [including ordinary cementite (Fe ₃ C) as well as cementite in which an alloying element such as Cr is dissolved as a solid solution] is 10
By controlling to ~ 50 nm, high strength and high toughness
It was found that ductility was achieved. Also at this time, the preferable range of the average particle size of the carbide is 10 to 30.
nm. From this, the structure of the steel wire in the present invention is not limited to fine pearlite, and includes pseudo pearlite and bainite, and any structure having one or more of these structures as the main constituent may be used. The remaining structure other than the main structure is not particularly limited, and examples thereof include ferrite.

The above results are for a steel wire having a wire diameter of 5 mm, and it is known that the mechanical properties of the steel wire, particularly the twisting property, depend on the wire diameter. It is expected that the appropriate range also depends on the wire diameter. In fact, in the above results, when the average particle size of cementite (including the above-mentioned “carbide”, the same applies hereinafter) becomes 10 nm or less, the strength becomes rather too high and vertical cracking starts to occur (see FIG. 1). , 5, 6), and an ultrafine wire having a wire diameter of 0.2 mm, vertical cracking did not occur even if the average particle size of cementite was less than 10 nm.

The reason why the above-mentioned phenomenon occurs is not entirely clear, but according to the studies by the present inventors, the following can be considered. Wire diameter is 0.2m
When comparing the dark field images of m and 5 mm steel wires when the average grain size of cementite is about the same, cementite is uniformly fined over the entire surface of the steel wire of 0.2 mm diameter. However, in the steel wire with a wire diameter of 5 mm, the cementite was only partly superfine, and the grain distribution as a whole was non-uniform. That is, even if the average particle diameter of cementite is almost the same, the problem of vertical cracking occurs even with ultra-fine graining because the ultrafine wire with a wire diameter of 0.2 mm has a relatively uniform grain size distribution. Not considered. Based on the above findings, the present inventors have further conducted intensive studies on the influence of the average grain size of cementite on the mechanical properties of steel wire while also considering the wire diameter dependency.

However, as the inventors proceeded with the above research, they encountered the following inconveniences. That is, in order to evaluate the ultrafine structure of lamella cementite, it is necessary to observe the actual structure as much as possible without depending on the formula (5), but the structure is completely evaluated by TEM observation using a normal thin film. I couldn't do that. In the ordinary extraction replica method or the like, lamella cementite may be mechanically damaged during handling of the sample, and there is no guarantee that the finally obtained carbide remains the structure before extraction. Even if you try to evaluate the ultrafine structure of lamella cementite using the ordinary extraction residue method, if the ultrasonic cleaning performed in the ordinary extraction residue process is performed, the external shape of cementite will collapse, and the internal structure will also be destroyed. It was mechanically damaged and significantly different from the pre-extraction tissue. In addition, the charcoal remaining after extraction without T
As a result of EM observation, when the crystal grains were 20 nm or less, the ultrafine structure was very difficult to discriminate, and it was not possible to discuss the relationship between the ultrafine structure and mechanical properties. Furthermore, when the accelerating voltage was increased to increase the resolution with the TEM, the extracted carbide was different from the structure before extraction due to irradiation damage due to the electron beam. For this reason, an attempt was made to evaluate using a field emission TEM capable of obtaining high resolution without increasing the accelerating voltage, but it was difficult to evaluate when the ultrafine structure was 10 nm or less.

Therefore, the present inventors first examined the means for analyzing the hyperfine structure from various angles. As a result, when the crystal grains were evaluated by using radiation, it was found that a detailed analysis can be performed even for an ultrafine structure having crystal grains of 20 nm or less. In addition, since the plate thickness of lamella cementite is as very small as several tens of nm, a sufficient integrated intensity could be obtained regardless of the absorption coefficient of radiation for the material. For this reason, various types of radiation can be used in this ultrafine structure analysis method. X-rays, for example, are most easily obtained as this radiation, but there is no problem in using the other radiations in the sense that structural analysis is performed.

The inventors of the present invention have applied the above-mentioned ultrafine structure analysis method to carefully examine the influence of the wire diameter R and the average particle diameter D of cementite on the mechanical properties of the steel wire including the ultrafine crystal grain region. Conducted a survey. The result is shown in FIG. 7.

As is apparent from FIG. 7, in order to obtain a steel wire having desired characteristics without causing (TS × elongation) deficiency and without causing vertical cracking, the above formula (1) should be satisfied. You can see that Further, as can be seen from FIG. 7, the average particle diameter D of cementite may be less than 10 nm or more than 50 nm as long as the expression (1) is satisfied. From FIG. 5, when the average particle diameter D is 30 nm or less (when the wire diameter R is 5 mm) (TS
Since the value of (elongation) is excellent, it is understood that it is preferable to satisfy the formula (2).

Next, the manufacturing conditions specified in the present invention will be described. In the method of the present invention, the true strain ε in wire drawing is set to 1.0 to 5.0 and the final soaking temperature T is set as the conditions for producing the high strength, high toughness and ductile steel wire as described above.
_B is operated at 700 ° C or lower. First, as a wire drawing material or a bluing material, the true strain ε must be 1.0 or more in order to obtain sufficient strength. Further, if the wire is excessively drawn, the strength is excessively increased and vertical cracking occurs, so that it is necessary to set the upper limit of the true strain ε to 5.0.

On the other hand, regarding the final soaking temperature T _B , if this temperature exceeds 700 ° C., depending on the steel type, γ transformation occurs and the fine and oriented pearlite structure obtained by wire drawing collapses. Since the mechanical properties deteriorate, the upper limit was set to 700 ° C. The final soaking temperature T
_{When B} is 500 to 700 ° C, spheroidization can be ignored for a few seconds, and tensile strength, elongation, drawing, twisting value, etc. do not drop extremely, but if it is exposed to this temperature range for a long time carelessly, it will become spherical. Can not be ignored, tensile strength, elongation, drawing,
The twist value etc. may be extremely reduced. On the other hand, if the temperature is lower than 300 ° C, the ductility may not be sufficiently recovered by the heat treatment. Therefore, the preferable range of the final soaking temperature T _B is about 300 to 500 ° C.

As shown in Tables 1 to 7, the present inventors have arranged the correlation between various manufacturing conditions and the average particle diameter D of cementite by using multiple regression analysis or the like. As a result, it was found that the average particle size defined by the above formula (3) has a clear correlation with the average particle size D of the cementite obtained by the measurement. That is, in order to obtain a desired cementite average particle diameter D according to the wire diameter R, the Si content in the steel wire and the average heating rate are set so that the average particle diameter D satisfies the above expression (3). It turned out that it is sufficient to control H _R.

FIG. 8 shows the relationship between the Si% when the wire diameter R is 5 mm, the average heating rate H _R and the average particle diameter D (the numeral in the figure indicates the average particle diameter D). And the above (1)
The area that satisfies the formula is shown by the dotted line. Also, FIGS.
[Fig. 3] is a diagram when the wire diameters R are 0.2 mm and 1.0 mm, respectively. Incidentally, these results are based on the data in Tables 1 to 7 of Examples described later. From these figures, each wire diameter R
Accordingly, there is an optimum range for the average particle size D, and when this average particle size D is arranged in relation to the manufacturing conditions, it becomes as shown in the above formula (3).

In the steel wire of the present invention, the structure mainly composed of fine pearlite can be achieved by performing hot-rolling and then directly patenting treatment or re-austenizing and then patenting treatment. . Further, the structure of pseudo pearlite or bainite is almost the same as the structure of pearlite, but the temperature during the patenting treatment may be adjusted appropriately.

The present invention will be described in more detail with reference to the following examples. The following examples are not intended to limit the present invention, and the present invention can be carried out with appropriate modifications within a range compatible with the gist of the preceding and the following. It goes without saying that all of them are within the technical scope of the present invention.

[0036]

【Example】

Example 1 High carbon steels having the chemical composition shown in Table 1 below are sample steels (AE).
After being hot-rolled into a steel wire having a diameter of 11 to 14 mm, it was subjected to lead patenting treatment and then cold-drawn at a predetermined working rate. The lead patenting treatment conditions at this time are: reheating 950 ° C. × 5 minutes → constant temperature transformation 540 to 540
It is 500 ° C. × 4 minutes.

[0037]

[Table 1]

Next, the steel wire is 5.0 mm which is the target wire diameter.
Continuous wire drawing was performed up to (area reduction rate: 71.0 to 87.2).
At this time, the steel wire is cooled at the die exit to keep the steel wire temperature at 17
The temperature was kept below 0 ° C. After that, linear processing is performed, and in order to simulate the temperature rising process during plating, 300 to 500 ° C
It was subjected to bluing treatment. Table 2 shows the average grain size and main structure of the carbide in the obtained steel wire together with the bluing treatment temperature. Table 3 shows the mechanical properties of each steel wire.

[0039]

[Table 2]

[0040]

[Table 3]

The following can be considered from Tables 1 to 3. First, the sample steel A is a steel type in which 1.21% of Si is added, and the pearling steel wire (N
o. In Nos. 1 and 2, the average particle diameter of the carbide (nano-cementite) is less than 10 nm, and therefore, twisting failure occurs with vertical cracking. Of these, the bluing temperature is 30
In the 0 ° C. pearlite steel wire (No. 1), the value of (TS × elongation) was also 8.0 kgf / mm ² , and the strength-elongation balance was lowered. The bluing temperature is 5
In the 00 ° C. pearlite steel wire (No. 7), the average grain size of carbides was coarsened to 51.8 nm, and (TS × elongation) was deteriorated. On the other hand, even if the same sample steel A is used, the pearlite steel wire (No. 3 to 6) having a bluing treatment temperature of 400 to 475 ° C. has an average grain size of carbides within an appropriate range defined by the present invention. Since it is within the range, (TS × elongation) shows an excellent value.

On the other hand, steel wire No. Nos. 8 to 10 were manufactured to have a bainite structure by using the test steel A, but in the bainite steel wire (No. 8) having a bluing treatment temperature of 300 ° C., an average of carbides (nano cementite) was obtained. Since the particle size is less than 10 nm, vertical splitting occurs and (TS
× elongation) is also inferior. The bluing temperature is 5
In the bainite steel wire (No. 10) at 00 ° C., the average grain size of carbides is coarsened to 52.8 nm, and the value of (TS × elongation) is deteriorated. On the other hand, even if the same test steel A is used, the average grain size of the carbide in the bainite steel wire (No. 9) having a bluing treatment temperature of 425 ° C. is within the appropriate range defined by the present invention. , (TS × elongation) show excellent values.

The sample steel B is a steel type to which Si is not added, and in the pearlite steel wire (No. 11) having a bluing treatment temperature of 300 ° C., the average grain size of carbides (nano cementite) is less than 10 nm. Therefore, the value of (TS × elongation) is deteriorated. Also, the bluing treatment temperature is 450
In the pearlite steel wire (No. 13) at ℃, the average grain size of the carbides was coarse and was 50.9 nm (TS × elongation).
The value of has deteriorated. On the other hand, even if the same test steel B is used, the pearlite steel wire (No. 12) having a bluing treatment temperature of 350 ° C. has an average grain size of carbides within the appropriate range specified in the present invention. , (TS × elongation) show excellent values.

Specimen Steel C is a steel type to which 0.29% of Cr is added, and in the pearlite steel wire (No. 14) having a bluing treatment temperature of 350 ° C., the average grain size of carbides (nano cementite) is 10 nm. Since it is less than 1, the twisting failure is accompanied by vertical cracking. Further, in the pearlite steel wire (No. 17) having a bluing treatment temperature of 500 ° C., the average particle size of cementite was coarse and was 50.8 nm,
The value of (TS x elongation) is deteriorated. On the other hand, even if the same test steel C is used, the bluing treatment temperature is 400 to
The 450 ° C. pearlite steel wire (Nos. 15 and 16) has an excellent value of (TS × elongation) because the average grain size of carbides is within the appropriate range specified in the present invention.

Specimen Steel D is a steel type in which C and Si are reduced, and in the pearlite steel wire (No. 18) having a bluing treatment temperature of 300 ° C., carbide (nano cementite) is used.
The average particle size was less than 10 nm and vertical cracking did not occur, but the value of (TS × elongation) was deteriorated. In addition, the pearling steel wire (No. 2) having a bluing treatment temperature of 450 ° C.
In 1), the average grain size of the carbides is coarse, 53.8 nm, and the value of (TS x elongation) is deteriorated. On the other hand, even if the same test steel D was used, the pearling steel wire (No. 1) having the bluing treatment temperatures of 400 ° C. and 425 ° C.
9 and 20), (TS × elongation) shows an excellent value.

The test steel E is a high C-added steel grade having a C content of 0.97%, and in the pearlite steel wire (No. 22) having a bluing treatment temperature of 300 ° C., the average grain size of carbides (nano cementite) is The diameter is less than 10 nm, and the twisting is accompanied by vertical cracks. The bluing temperature is 5
In the pearlite steel wire (No. 25) of 00 ° C., the average grain size of carbides is coarsened to 53.1 nm, and the value of (TS × elongation) is deteriorated. On the other hand, even when the same test steel E was used, the bluing treatment temperatures were 425 ° C. and 45
For pearlite steel wire (No. 23, 24) at 0 ° C, (T
S × elongation) shows an excellent value.

Example 2 Steel type A drawn to a wire diameter of 5 mm by the process described above,
Regarding B, it was further subjected to lead patenting (lead patenting conditions: reheating 950 ° C. × 5 minutes → constant temperature transformation 540 to 500 ° C. × 4 minutes). The target wire diameter of these steel wires is 1.0 mm (area reduction ratio: 71.0 to 87.2).
%) Was continuously drawn. At this time, the wire rod was cooled at the outlets of all the dies, and the wire rod temperature was maintained at 170 ° C or lower. After that, linear processing was performed and bluing treatment was performed at 300 to 500 ° C.

Subsequently, in the same manner as above, lead patenting was performed, brass plating was applied, and then wire drawing was performed. These steel wires were wet-drawn continuously to a target wire diameter of 0.2 mm (area reduction ratio: 71.0 to 87.2%). Then, after direct processing, bluing in the range of 300 to 500 ° C was performed.

The average grain size and main structure of the carbide in the obtained steel wire are shown in Table 4 together with the bluing treatment temperature. Table 5 shows the mechanical properties of each steel wire. Tables 4 and 5 also show the results for steel type B when the wire diameter was 1.0 mm.

[0050]

[Table 4]

[0051]

[Table 5]

The following can be considered from Tables 4 and 5. No.
Nos. 26 to 28 are steel types A drawn to 0.2 mm. When the wire diameter is 0.2 mm, the lower limit of the average particle diameter D obtained by the equation (3) is 3.1 nm (Fig. 9), while the average particle diameter is 300 ° C for the bluing treatment temperature. Since D is as small as 2.8 nm, vertical cracking has occurred. In the case of the material treated at 500 ° C., the upper limit of the average particle diameter obtained by the equation (3) is 43.1 nm (FIG. 9), but since it is as large as 54.3 nm, (TS × elongation) is 6.1 kgf / It was less than mm ² and 9.6 kgf / mm ² .

No. 29 to 31 have steel type B of 0.
It is drawn to 2 mm. Since the material treated at 300 ° C satisfied the formula (3) (Fig. 9), there was no vertical crack, and (TS x elongation) was 12.0 kgf / mm ² and the lower limit value was 9.6 kgf / mm ² or more. there were. In the 450 ° C. treated material, the particle size is the upper limit value calculated by the equation (3) 43.
Since 14.3 nm (Fig. 9) is as large as 54.3 nm,
(TS x elongation) is 5.3 kgf / mm ² and the lower limit value is 9.6.
It was below kgf / mm ² .

No. Steel types B of 32-36 are 1.0
The wire is drawn up to mm. The particle size of the 300 ° C. treated material is 46.6 nm, which is the upper limit value obtained by the equation (3).
As compared with (FIG. 10), vertical cracks were generated because it was as small as 4.8 nm. Also, (TS x elongation) is 8.3 kgf / m
It was below m ² and 9.6 kgf / mm ² .

In the 500 ° C. treated material, the particle size is as large as 50.8 nm as compared with the lower limit value of 46.6 nm (FIG. 10) obtained by the equation (3), so (TS × elongation) is 4.
The 9kgf / mm ² and 9.6kgf / mm ² were below.

Example 3 When the wire diameter was drawn up to 1.0 mm under the same conditions as described above, the heating rate and the wire drawing strain (true strain ε) were changed, and the right side of the equation (3) was changed. The case where the value of was changed was investigated. Table 6 shows the average grain size and main structure of the carbide in the obtained steel wire together with the bluing treatment temperature. Table 7 shows the mechanical properties of the steel wire.

[0057]

[Table 6]

[0058]

[Table 7]

From these results, the following can be considered.
First, the case where the wire drawing rate is changed will be described. Even when the bluing temperature is 350 ° C, the wire drawing strain is 2.
It is reduced from 2 to 1.6, and the value on the right side of equation (3) is 5.
When the wire diameter is set to be 2 or less and the wire diameter is 1.0 mm or less, the (TS × elongation) becomes 8.8 kgf / mm ² and the (TS × elongation) becomes insufficient, and the average particle diameter is 4. 9n
m or less. Even in the case of 425 ° C. bluing treatment, the wire drawing strain was increased from 2.2 to 3.2 so that the value on the right side of the equation (3) was 55.4 nm and the wire diameter was 1.0 m.
If it is more than the upper limit of m, (TS x elongation) is 8.4k.
gf / mm ² (TS × elongation) is insufficient, and the average particle diameter D is 53.9 nm or more.

Next, the case where the average heating rate H _R is changed will be described. Here the final soak temperature is taken as T _B, H _R is the average heating rate between T _B -20 ° C. from T _B -100 ° C.. Soaking temperature 45 bluing treatment temperature
Aligning with 0 ℃, heating rate from 12.6 ℃ / sec to 1.0 ℃
When it is slowed down to / second, the value on the right side of the expression (3) is 66.4, which exceeds the upper limit value when the wire diameter is 1.0 mm. in this case,
(TS x elongation) becomes 8.4 kgf / mm ² (TS
X elongation) is deteriorated. The heating rate is 126.0 ° C.
It can be seen that when the speed is increased to / second, the value on the right side of the equation (3) becomes 29.4, which is within the appropriate range for the wire diameter of 1.0 mm, and the mechanical properties are excellent.

[0061]

The present invention is constituted as described above, and by controlling the average grain size of the carbide of the structure in the steel wire to an appropriate range at the nanometer level, the desired high strength and high ductility steel can be obtained. Wire can be obtained, this steel wire is PC
It is most suitable as a material for steel wire, galvanized steel wire, steel wire for springs, cables for suspension bridges, etc.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the average grain size of cementite in a steel wire having a wire diameter of 5 mm and (TS × drawing).

[Fig. 2] For steel type A, the bluing temperature is 425 ° C.
It is a drawing-substituting micrograph showing the metal structure at the time.

[FIG. 3] In steel type A, the bluing temperature is 475 ° C.
It is a drawing-substituting micrograph showing the metal structure at the time.

[FIG. 4] In steel grade A, the brewing temperature is 300 ° C.
It is a drawing-substituting micrograph showing the metal structure at the time.

FIG. 5 is a graph showing the relationship between (TS × elongation) and the average particle size of cementite in a steel wire having a wire diameter of 5 mm.

FIG. 6 is a graph showing the relationship between (TS × twist value) and the average particle size of cementite in a steel wire having a wire diameter of 5 mm.

FIG. 7 is a graph showing the influence of the wire diameter R and the average particle diameter D of cementite on the mechanical properties of the steel wire.

FIG. 8 is a graph showing the relationship among the amount of Si, the heating rate, and the average particle diameter D when the wire diameter is 5 mm.

FIG. 9 is a graph showing the relationship among the amount of Si, the heating rate, and the average particle diameter D when the wire diameter is 0.2 mm.

FIG. 10 is a graph showing the relationship among the amount of Si, the heating rate, and the average particle diameter D when the wire diameter is 1.0 mm.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koichi Makii 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Hyogo Prefecture Kobe Steel Works, Ltd., Kobe Research Institute (72) Inventor Atsushi Miyamoto Kobe City, Hyogo Prefecture 1-5-5 Takatsukadai, Nishi-ku Kobe Steel Works, Ltd., Kobe Research Laboratory (56) References JP-A-50-91517 (JP, A) JP-A-3-215626 (JP, A) JP-A 4-346618 (JP, A) JP-A-6-235023 (JP, A) JP-A-4-236742 (JP, A) JP-A-4-325627 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 301 C21D 9/56 102

Claims (9)

(57) [Claims] 1. A steel wire mainly composed of at least one kind of structure selected from the group consisting of fine pearlite, pseudo-pearlite and bainite, wherein the average grain size of carbides in the structure is 10 to 50 nm. High strength, high toughness and ductile steel wire. 2. A steel wire mainly composed of a fine pearlite structure, wherein the average grain size of the cementite crystal grains in the lamella cementite constituting the fine pearlite is 10 to 50 nm.
The high-strength, high-toughness / ductile steel wire according to claim 1. 3. The high-strength, high-toughness / ductile steel wire according to claim 1, which has an average particle diameter of 10 to 30 nm. 4. A steel wire mainly composed of at least one structure selected from the group consisting of fine pearlite, pseudo-pearlite and bainite, wherein the average grain size D of carbide in the structure is D.
(Nm) and wire diameter R (mm) satisfy the relationship of the following formula (1), a high strength and high toughness ductile steel wire. 2.14 × lnR + 6.56 ≦ D ≦ 2.14 × lnR + 46.6 (1) 5. The average particle diameter D (n ) of carbides in the structure
The high-strength, high-toughness / ductile steel wire according to claim 4, wherein m) and the wire diameter R (mm) satisfy the relationship of the following expression (2). 2.14 × lnR + 6.56 ≦ D ≦ 2.14 × lnR + 26.6 (2) 6. A steel wire mainly composed of a fine pearlite structure, wherein the average grain size D of the cementite crystal grains in the lamellar cementite constituting the fine pearlite satisfies the above formula (1) or (2). Or claim 4
High strength, high toughness and ductile steel wire described in. 7. When producing the high-strength, high-toughness / ductile steel wire according to any one of claims 1 to 6, the true strain ε in wire drawing is set to 1.0 to 5.0, and final soaking is performed. A method for producing a high-strength, high-toughness / ductile steel wire, which comprises operating at a temperature T _B of 700 ° C. or lower. 8. The manufacturing method according to claim 7, wherein the final soaking temperature T _B is 300 to 500 ° C. 9. The manufacturing method according to claim 7, wherein the operation is performed so that the average particle diameter D (nm) satisfies the following expression (3). D = -108.7-12.9 × [Si] + 16.4 × ε + 0.320 × T B -17.6 × logH R ... (3) where, [Si]: Si content in the steel wire ( wt%) H _R: average heating rate from T _B -100 ° C. until between T _B -20 ℃ (℃ / sec)