JP3292504B2 - High carbon steel wire with good workability - Google Patents

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 suitable as a material for a high strength steel wire such as a steel cord, a bead wire, a PC steel wire, etc., and particularly to a wire having good workability. What you want to do.

[0002]

2. Description of the Related Art High carbon steel wires are used for various applications requiring high strength, such as PC steel wires, wire ropes, bead wires, and tire cords. In recent years, for these steel wires, in order to reduce costs or improve productivity, a high-speed drawing technique and a technique of omitting patenting during drawing have been put to practical use. On the other hand, the need for higher tensile strength of steel wire has been increasing year by year. For example, Japanese Patent Application Laid-Open No. 2-197524 discloses a method of imparting a large working strain to a material to which a predetermined alloy element has been added for an ultrafine wire. A method for increasing the tension is disclosed. This method combines the addition of a specific alloying element and the heat treatment conditions to achieve a finer and more ordered pearlite structure to produce a high-strength steel wire with high toughness, and further increases the cold-drawing strain to achieve the final It is to increase the strength.

[0003]

In general, the final strength of a steel wire largely depends on the processing strain, and the higher the processing strain, the higher the strength. On the other hand, as the strength increases, the ductility tends to deteriorate. That is, at the time of wire drawing for imparting processing strain, microcracks tend to be generated because the central portion of the steel wire receives high tensile stress in the die, and even if the microcracks at the central portion of the steel wire do not lead to disconnection, the steel wire may be broken. This causes the wire ductility and fatigue characteristics to decrease. In addition, C,
The presence of segregation of Mn, P, and the like further promotes the above-mentioned phenomenon, resulting in a decrease in ductility and breakage. It is considered that these phenomena become more apparent as the degree of processing increases.

[0004] Therefore, in order to obtain a high strength steel wire by adjusting the strength and ductility after heat treatment and imparting a high workability, it is necessary to control the microstructure at the center of the steel wire, in other words, to control the steel material as the material. Control of the microstructure of the part is very important. The present invention can solve the above-described problems and maintain high strength and ductility after drawing by solving the results of research and development on the relationship between the microstructure of the wire center and the drawability.
An object is to propose a high-carbon steel wire rod with good workability.

[0005]

According to the present invention, the C content is reduced.
A 0.75 wt% or more of the high carbon steel wire rod, the axis of該線material
Ferrite area ratio of 0.5 to 10% at the center of cross section orthogonal
And this region is 1 to 1 of the total cross-sectional area.
It is a high-carbon steel wire with good workability characterized by being 5%.

[0006]

First, the component composition of the present invention will be described. C is an important component for obtaining a desired strength after the final wire drawing, and in order to exert its effect sufficiently, the content of 0.75 wt% or more is required. The upper limit of the content is not particularly limited, but the higher the content, the higher the strength of the steel wire. However, if it is excessively increased, the steel wire becomes brittle due to the influence of reticulated cementite and the like, so the upper limit is about 1.0%.

[0007] When the alloy component needs to be contained,
i, Mn, Cr, V and the like may be appropriately contained. In this case, the content is as follows: Si: about 0.1 to 1.5 wt%, Mn: 0.3 to 1.5 wt%
%, Cr: 0.1 to 0.5 wt%, V: 0.05 to 0.30 wt%
It is about. Further, from the viewpoint of preventing ductility deterioration, P and S of the impurity components are preferably suppressed to 0.010 wt% or less, respectively, and O is desirably 30 ppm or less in consideration of a reduction in the amount of inclusions.

Next, the microstructure at the center of the wire rod will be described together with the experimental results on which the present invention is based. When the C content is 0.75 wt% or more, most of the microstructure after hot rolling or patenting during wire drawing is a pearlite structure even from the Fe-C phase diagram, and the ferrite structure is Almost no precipitation. In addition, C, Mn and the like segregate at the center, and for example, in the case of C, the concentration becomes 1.1 to 1.3 times, so that the center has a finer pearlite structure than the matrix. On the other hand, C content is 0.75wt%
When the content is less than the above, it is composed of a mixed structure of ferrite and pearlite, and the lower the C content, the higher the ferrite fraction and the lower the pearlite fraction.

Although it is clear that the microstructure changes depending on the C content as described above, the effect of the wire drawing workability accompanying such a change in the microstructure was examined in detail. In the test steel, the amount of C was changed in the range of 0.5 to 0.97 wt% in order to make the ratio of ferrite various, and the amount of Mn was made into a 100 kg ingot made by vacuum melting with a constant amount of 0.55 wt%. 5.5 mm by hot rolling
It was rolled to φ wire. The rolling conditions are all the same,
At the time of cooling, steermore adjusted cooling was performed. Using the obtained various wires, accelerated drawing was performed using a die having an approach angle of 20 ° with a half angle of the die larger than usual.

[0010] At that time, the limit of wire drawing (disconnection generation strain)
The workability was evaluated by the following formula, and the relationship with the ferrite area ratio is shown in FIG. Here, the ferrite area ratio is obtained by arbitrarily observing 10 fields of view in a cross section of 5.5 mmφ by SEM, and then measuring with an image analyzer. As is evident from FIG. 1, the processing limit tends to increase as the ferrite area ratio increases. Even at a ferrite area ratio of about 0.7%, the processing limit greatly increases as compared with a sample having a ferrite area ratio of 0%. The processing limit gradually increases as the ferrite area ratio increases. It is considered that the reason for this is that, since ferrite has higher deformability than pearlite, even if a very small amount of ferrite is present, the drawing is improved.

By the way, if such a ferrite structure exists in the central part of the steel material, good workability can be obtained as a whole wire. Therefore, the lower limit of the ferrite area ratio at the center of the cross section orthogonal to the axis of the wire (hereinafter, simply referred to as the center of the wire) is set to 0.5% as the minimum amount for obtaining good workability. On the other hand, when the amount of ferrite increases,
Since the strength decreases, the upper limit of the ferrite area ratio is 10
%.

[0012] For this intensity, it is considered that both the proportion occupied in the entire area having the area ratio of ferrite and its ferrite area ratio of the wire center (occupancy) affects, then predetermined ferrite area ratio described above Territory with
An experiment for examining the effect of the area occupancy (hereinafter, simply referred to as ferrite occupancy) was performed as follows. The amount of C in the substrate is 0.
Prepare a 150mm square bar of 85wt%, and in the center of this square bar, adjust the amount of C to adjust the ferrite area ratio to 7 ~ 8%, and change the area occupying the entire cross-sectional area of the square bar in various ways. By hot-rolling to 5.5 mmφ, many samples with various ferrite occupancy ratios are placed in the center of the 5.5 mmφ wire.
I prepared a few . In addition, since the effect of the longitudinal direction or the wire diameter appears on the strength as it is rolled, the diameter of the wire rod is increased from 5.5 mmφ to 5.0 mm.
The wire was drawn to mmφ to make the wire diameter uniform, then austenitized at 920 ° C. for 10 minutes, and air-cooled to conduct a tensile test. The result is shown in FIG.

As shown in FIG. 2, when the ferrite occupancy is in the range of 0 to 5%, the strength hardly changes, and when the ferrite occupancy exceeds 5%, the strength decreases. Therefore, in consideration of the processing limit, the lower limit of the ferrite occupancy is set to 1%, and the upper limit is set to 5% at which the strength does not decrease.

To produce the high carbon steel wire of the present invention,
As an example, there is a method in which, when continuously casting molten steel having a suitable component composition, forging is performed in the vicinity of a crater end where internal molten steel of a slab completes solidification to adjust the C concentration in the central portion of the steel material. The method for producing a steel material according to the present invention is not limited to the above example. In short, any manufacturing method may be used as long as a region having a ferrite structure at a predetermined ratio in the center of the steel material can be obtained.

[0015]

EXAMPLE A molten steel of high carbon steel having the chemical composition shown in Table 1 was prepared.

[0016]

[Table 1]

This molten steel is subjected to continuous casting by 400 mm × 560 mm.
At the end of solidification, a large pressure reduction (maximum reduction amount of 150 mm) is performed by a die at the end of solidification, and the concentrated molten steel at the center of the slab is discharged to the non-solidified side (anti-casting direction side) to complete solidification. A point was formed. The composition and region of the central portion of the slab were variously adjusted by controlling the unsolidified thickness and the amount of reduction during rolling by adjusting the casting speed. These slabs are hot-rolled to 15
After passing through a 0 mm square billet, it was turned into a wire of 10 mmφ.
The film was wound at 0 ° C., and then subjected to stealmore cooling. From the 10 mmφ wire thus obtained, the mechanical properties of 20 consecutive samples were examined, and the results are shown in Table 2.

[0018]

[Table 2]

In Table 2, symbols A, B and C are steels of the present invention. D is the same as the conventional casting condition (no reduction was applied), so there was no ferrite area ratio and occupancy, and conversely, positive segregation occurred. E: Ferrite area ratio exceeded 10% , F is 5% ferrite occupancy
G exceeds the upper limit of the present invention in both of the ferrite area ratio and the occupancy.

As is clear from Table 2, the steel of the present invention has a strength of 120 kgf / mm ² or more and a high drawing value of 40%. On the other hand, Comparative Steel D has the same strength level as the steel of the present invention, but has a low drawing of 33%. This is because there is positive segregation in the center, and network cementite is precipitated. E, F and G have a higher aperture value, but all have a strength of 4 to 5 kgf / mm compared with the steel of the present invention.
² low.

Next, the as-rolled steel materials of the present invention steels A to C and comparative steels D to G shown in Table 2 were pickled, pre-treated, cold-drawn in 8 passes, and then subjected to blueing treatment. Thereafter, a stranded wire process was performed to experimentally produce a PC steel stranded wire (SWPR 7B, 12.7 mm from seven wires) specified in JIS G3536. Table 3 shows the mechanical properties and relaxation values of the 4.2 mmφ steel wire after drawing. In the relaxation test, a load corresponding to 80% of the minimum value of the load with respect to the 0.2% permanent elongation was applied at room temperature, and thereafter, the reduction of the load was measured by holding the grip for 10 hours. % Or less shall be within the standard.

[0022]

[Table 3]

As is evident from Table 3, A, B and C using the steel of the present invention have sufficiently high strength and ductility, and also have sufficiently low relaxation values, showing excellent properties. On the other hand, D using the comparative steel had sufficient strength but also low ductility, and suffered one cutpy break during wire drawing due to network cementite. E, F and G using the comparative steels inherit the properties of the wire and have good ductility but low strength. Furthermore, although the relaxation value is within the standard, it shows a higher value, and concerns remain in obtaining stable quality over the entire length.

From the above, the steel of the present invention can obtain excellent properties not less than the specified value even if the patenting before drawing is omitted, but the comparative steel performs patenting before drawing, It is necessary to prevent precipitation of reticulated cementite and increase strength. It is obvious that if the steel of the present invention is subjected to a patenting treatment, higher strength and higher ductility than conventional steels can be obtained.

[0025]

According to the wire rod of the present invention, by controlling the microstructure of the central part of the high carbon steel wire rod, the strength can be maintained as it is rolled and the ductility can be increased. Have. In addition, it is also possible to omit the heat treatment before drawing or during the drawing depending on the use, and there is an effect that the manufacturing cost can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is a graph showing an influence of a ferrite area ratio on a limit of wire drawing workability.

FIG. 2 is a graph showing the effect of ferrite occupancy on tensile strength.

Claims (1)

(57) [Claims] 1. A high carbon steel wire having a C content of 0.75% by weight or more, comprising a region having a ferrite area ratio of 0.5 to 10% at a center portion of a cross section orthogonal to an axis of the wire, and This region is 1 to 5% of the total cross-sectional area, and a high-carbon steel wire with good workability,