KR20190099858A - Method and apparatus for manufacturing steel wire - Google Patents

Abstract

The present invention relates to a method for manufacturing a steel wire and a device for the same comprising a first drawing step for drawing a wire rod through a first die, a ball rolling step for rolling the wire rod having passed through the first die through at least one ball rolling roll, and a second drawing step for drawing the wire rod having passed through the ball rolling roll through a second die, wherein the wire rod is drawn out at a cross-sectional reduction rate in a range of 10-40% in the first drawing step, is rolled at a cross-sectional reduction rate in the range of 10-40% in the ball rolling step, is drawn out with a cross-sectional reduction rate in the range of 10-40% in the second drawing step, and is finally processed such that the cumulative cross-sectional reduction rate of the wire rod is 50% or more. Therefore, the present invention is capable of improving a corrosion resistance of the steel wire.

Description

Method and apparatus for manufacturing steel wire {Method and apparatus for manufacturing steel wire}

The present invention relates to a method and apparatus for manufacturing steel wire for drawing steel wire by drawing and rolling the wire, and more particularly, the wire can be processed at a predetermined cross-sectional reduction rate in order to secure the dislocation density required for grain refinement. The present invention relates to a steel wire manufacturing method and apparatus capable of producing steel wire with improved corrosion resistance.

In order to overcome the recent tightening of fuel economy regulations globally, it is necessary to reduce the weight of automobile parts, and for this purpose, high-strength materials are essential. However, when the material is used in a high strength region, there is a problem that the corrosion resistance is limited by the use environment.

Specifically, as the material is strengthened, corrosion sensitivity increases, and corrosion in high-strength steel is mostly pitting corrosion, which is a notch sensitivity due to grain dropping in which the entire grain is dropped by grain boundary corrosion. Is increased so that the crack propagates.

In general, an alloying element is added to a material to improve corrosion resistance, which causes a problem of rising material cost.

Austenitic grain boundary properties are affected by corrosion resistance due to grain boundary angle, coherent phase, and crystal orientation misalignment, and according to these characteristics, random boundary and coincidence site lattice (CSL) It is divided into boundaries.

1 is a conceptual diagram showing the effect of the austenite grain boundary characteristics on the corrosion resistance of the material. Referring to this, the more the random boundary, the lower the corrosion resistance, and the more the Coincidence Site Lattice (CSL) boundary, the better the corrosion resistance.

The refinement of austenite grains has the effect of increasing the number of Coincidence Site Lattice boundaries. Therefore, in order to improve the high strength and corrosion resistance without adding the alloying element to the material, the grains are refined.

On the other hand, in the manufacture of steel wire (steel), the diameter required by the customer through the drawing process of passing the die (die: 8 ~ 20mm) products produced in the steel mill wire rod factory by the tensile force (7 ~ 19mm) is made by the way of making products. That is, as the raw material (wire) having a circular cross section is drawn, a product having a reduced cross-sectional area and an increased dislocation density in the raw material (or simply sized product) is produced.

For example, in order to manufacture automotive suspension springs, hot forming and cold forming are used as processes using medium carbon steel as a drawing material. These austenitic heat treatments are conducted by mixing induction heating or air heating, and tempering, which is a heat treatment for softening martensite structure obtained after quenching, is also used in induction heating or atmospheric heating.

In general, the drawing material subjected to the induction heating heat treatment process is given a drawing process in the range of 15-25% reduction in cross-section, it is common to perform austenite heat treatment in the range of 950 ~ 980 ℃. This requires a very fast austenitization because the heating time is given within a few tens of seconds. For this purpose, when the induction heating temperature is increased or the induction heating temperature is lowered, an appropriate dislocation density must be secured to increase the diffusion speed of atoms. In the case of spring steel wire rods, the drawing density in the range of 15 to 25% reduction in cross section can ensure the dislocation density in the range of 1.8 to 2.5x10 ⁸ mm ^-2 .

When heat treatment, Ac ₃ (heating transformation point that becomes austenite single phase, usually around 820 ° C for spring steel) If austenite heat treatment temperature can be lowered to the temperature range directly above the transformation point, fine austenite grains do not occur after recrystallization Can be obtained.

The size of austenite grains varies considerably with the heating temperature to obtain austenite single phase. Figure 2 is a graph comparing the average size of austenite grains in the atmospheric heating conditions and induction heating conditions. Referring to this, it can be seen that the atmospheric heating conditions have an average grain size of austenite of 8 μm at 850 ° C., but 45 μm at 1000 ° C. In addition, induction heating conditions have an average grain size of austenite of 5 μm at 850 ° C., but can be seen to be 16 μm at 1000 ° C.

However, ferrite + pearlite, which is composed of the initial microstructure of the drawn material, is partially re-used as austenite during heat treatment and remains partially, resulting in deterioration of physical properties. For reference, FIG. 3 illustrates a phenomenon in which unemployed pearlite remains when the heat treatment temperature is lowered (850 ° C.). Therefore, the heat treatment temperature should be increased or the heat treatment time should be increased. As described above, when heat treatment is performed at a temperature range immediately above the Ac ₃ transformation point through a conventional drawing process, there is a problem in that physical properties decrease.

4 is a graph showing the required potential density for complete re-use of austenite under induction heating conditions. Referring to FIG. 4, in order to completely re-use ferrite + pearlite as austenite during heat treatment, In this case, a dislocation density of 4x10 ⁸ mm ^-2 or more is required, and for this purpose, the reduction rate of the cross section by cold working must satisfy 50% or more.

As described above, in order to refine the austenite grain size to a predetermined level (5 µm or less) in the induction heating heat treatment criteria, it is necessary that the austenite heat treatment is possible at a temperature range immediately above the Ac ₃ transformation point. In addition, in order to completely reconsider ferrite + pearlite (ferrite + pearlite) composed of the initial microstructure of the drawn material as austenite during heat treatment, a spring density of 4x10 ⁸ mm ^-2 or more is required for spring steel. Section reductions of at least 50% must be met.

On the other hand, when the spring steel material, which is a medium carbon alloy steel, gives a drawing amount of 25% or more, triaxial tensile stress higher than the yield strength occurs in the center of the material, and as a result, as shown in FIG. 5, chevron cracking during drawing is performed. (Chevron crack) frequency is very high, there is a problem that disconnection occurs during the drawing process.

In general, the limit of cross-sectional reduction rate of drawing is different depending on the steel type, but about 40% of low carbon steel, about 30% of medium carbon steel and alloy steel, and about 20% of high carbon steel and alloy steel.

In addition, even if the drawing process amount is more than 25%, there is a problem in that coarse micro shear bands are created due to mechanical instability of general drawing processing characteristics. For reference, FIG. 6 shows a comparison of micro-share bands generated at 40% and 20 to 25% of the amount of draw processing.

The formation of such coarse micro share bands may cause problems in quality when a large part of the processed tissue remains as the recrystallization rate is very slow during induction heating.

Furthermore, in order to achieve a drawing processing amount of 50% or more in cross-sectional reduction rate, the problem of surface defects caused by breakage of the lubricating film due to an increase in drawing processing pressure must be overcome.

Korean Patent Publication No. 10-2004-0035136 (published Apr. 29, 2004)

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and provides a steel wire manufacturing method and apparatus capable of securing a predetermined or more cross-sectional reduction rate in order to secure dislocation density required for grain refinement, thereby improving corrosion resistance of steel wire. Let it be technical problem.

According to one embodiment of the present invention, a method for manufacturing a steel wire according to the present invention includes a primary drawing step of drawing a wire rod through a first die; A ball rolling step of rolling the wire rod having passed through the first die through at least one ball rolling roll; And a second drawing step of drawing the wire rod passing through the ball rolling roll through a second die. The method for producing a steel wire according to the present invention draws the wire rod at a cross-sectional reduction rate in the range of 10 to 40% in the primary drawing step, rolls at a cross-sectional reduction rate in the range of 10 to 40% in the ball rolling step, and the secondary In the drawing step, the drawing is carried out at a cross sectional reduction rate in the range of 10 to 40%, and finally, the cumulative cross sectional reduction rate of the wire is characterized in that the processing is 50% or more.

The reduction rate of the cross section of the wire rod in the primary drawing step is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, in the range of 10 to 30% for 0.3 to 0.6% C medium carbon steel and its alloy steel, and 0.6 For high carbon steels and their alloy steels it may range from 10% to 40%.

The reduction rate of the cross section of the wire rod in the step of rolling the die is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, in the range of 10 to 30% for 0.3 to 0.6% C medium carbon steel and its alloy steel, and 0.6 to In the case of 1.0% C high carbon steel and its alloy steel, it may have a range of 10-25%.

The reduction rate of the cross section of the wire rod in the secondary drawing step is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, in the range of 10 to 30% for 0.3 to 0.6% C medium carbon steel and its alloy steel, and 0.6 For high carbon steels and their alloy steels it may range from 10% to 30%.

The wire rod that has completed the secondary drawing step may have a dislocation density of 4.0 × ^{10 8} mm ⁻² or more.

The method of manufacturing a steel wire according to the present embodiment may further include performing austenite heat treatment of the wire rod after completion of the secondary drawing step.

The austenitic heat treatment is carried out in the temperature range of Ac3 ~ Ac ₃ + 80 ℃ that is directly above the austenite transformation point (Ac ₃ )

During the austenite heat treatment, the heat treatment may be performed such that the undissolved pearlite has a fraction of 5% or less.

After the austenite heat treatment, the fraction of grain boundaries having a grain boundary of 15 ° or less may be 20% or more, and the average size of grain sizes may be 7 µm or less.

According to another embodiment of the present invention, a steel wire manufacturing apparatus according to the present invention, the first die for primary drawing the wire rod; One or more ball rolling rolls for ball rolling the wire rod passed through the first die; And a second die for secondary drawing of the wire rod having passed through the ball rolling roll. According to the steel wire manufacturing apparatus according to the present invention, the first die is configured such that the wire is drawn at a cross-sectional reduction rate in the range of 10 to 40%, and the ball rolling roll is rolled at a cross-sectional reduction rate in the range of 10 to 40%. And the second die is configured such that the wire is drawn at a cross sectional reduction rate in a range of 10 to 40%, and the first and second dies and the ball rolling roll have a cumulative cross sectional reduction rate of the wire 50 or more. It is composed.

The ball rolling roll, the elliptical ball rolling roll of the elliptical cross section through which the wire rod passes; And installed on the rear of the elliptical ball rolling roll, the wire rod may include a circular ball rolling roll of the circular cross section.

The first die has a cross-sectional reduction rate of the wire rod in the range of 10% to 40% for low carbon steel and its alloy steel of 0.3% C or less, 10% to 30% range for 0.3 to 0.6% C medium carbon steel and its alloy steel, and 0.6 to In the case of 1.0% C high carbon steel and its alloy steel, it may be configured to have a range of 10-40%.

The reduced rolling rate of the wire rod, the cross-sectional reduction rate of the wire rod is less than 0.3% C 10% to 40% range for low carbon steel and alloy steel, 0.3% to 0.6% C medium carbon steel and 10 to 30% range for alloy steel, 0.6 to In the case of 1.0% C high carbon steel and its alloy steel, it may be configured to have a range of 10-25%.

The second die has a cross-sectional reduction rate of the wire rod in the range of 10 to 40% for low carbon steels and alloy steels of 0.3% C or less, and 10 to 30% for 0.3 to 0.6% C medium carbon steels and alloy steels, and 0.6 to In the case of 1.0% C high carbon steel and its alloy steel, it may be configured to have a range of 10-30%.

According to an embodiment of the present invention, for wire rods of various wire diameters used in the steel industry 1) drawing material having a variety of wire diameters by having a different amount of drawing processing at a specific wire size, 2) grain refinement by steel drawing processing, 3 ) A method for obtaining advantages such as diversification of heat treatment conditions due to an increase in dislocation density. Delaying the time of the occurrence of the center crack generated when the amount of cold drawing is increased, thereby obtaining a manufacturing condition suitable for drawing steel. It has an effect.

By securing the steel wire manufacturing conditions, it is possible to produce a steel drawing without disconnection, thereby generating a dislocation density of 4.0x10 ⁸ mm ^-2 or more, and thereby low heat treatment temperature (Ac ₃ ~ Ac ₃ +80). By allowing the austenite single phase to be secured at 7 ° C., the austenite grains can be miniaturized to 7 μm or less, thereby significantly increasing the corrosion resistance of the steel wire.

1 is a conceptual diagram showing the effect of austenite grain boundary characteristics on the corrosion resistance of the material.
Figure 2 is a graph comparing the average size of austenite grains in atmospheric heating conditions and induction heating conditions.
3 is a conceptual diagram and micrograph showing whether the unemployed pearlite remains according to the heat treatment temperature.
Figure 4 is a graph showing the required potential density for complete re-use of austenite in induction heating conditions.
5 is a conceptual diagram showing the occurrence of Severon cracks according to the rate of cross-sectional reduction.
Figure 6 is a photograph showing that the micro shear band (micro shear band) occurred at 20% or more of the amount of pull out.
7 is a diagram showing a triaxial tensile stress distribution inside a material in a conventional drawing process.
8 is a view showing a steel wire manufacturing apparatus according to an embodiment of the present invention.
9 is a cross-sectional view showing a cross section of the wire rod passing through the ball rolling roll shown in FIG.
10 is a view comparing the strain distribution of the cross section of the raw material after drawing and the strain distribution of the cross section of the raw material after the ball rolling;

As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all transformations, equivalents, and substitutes included in the spirit and scope of the present invention. In the following description of the present invention, if it is determined that the detailed description of the related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted.

Terms such as first and second may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprises" or "having" are intended to indicate that there is a feature, number, step, action, component, part, or combination thereof described on the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Hereinafter, an embodiment of the steel wire manufacturing method and apparatus according to the present invention will be described in detail with reference to the accompanying drawings, in the description with reference to the accompanying drawings, the same or corresponding components are given the same reference numerals and Duplicate explanations will be omitted.

In the case of general spring steel, it is difficult to give a cross-sectional reduction rate of more than 30% when drawing, because chevron cracks are generated due to an increase in the triaxial stress in the center. 7 is a view for explaining the cause of the material disconnection in a conventional drawing process. FIG. 7 shows a result of predicting tri-axial tensile stress generated inside the material by using the finite element method as the material is plastically deformed inside the drawing die.

When triaxial tensile stress occurs, cracking occurs locally beyond the central tensile strength, or at the boundary between inclusions and matrix, even at tensile stresses lower than yield strength, if there are microscopic nonmetallic inclusions in the material. This is because the pores form, grow, and coalesce to grow into macroscopic cracks. As such, the triaxial tensile stress is a direct cause of disconnection of the material.

8 is a view showing a steel wire manufacturing apparatus according to an embodiment of the present invention, Figure 9 is a cross-sectional view showing a cross section of the wire passing through the ball rolling roll shown in FIG.

As shown in FIG. 8, the steel wire manufacturing apparatus according to the present embodiment is configured to carry out a nodular rolling of the first die 10 for primary drawing the wire rod 1 and the wire rod 1 passing through the first die 10. One or more ball rolling rolls 20 and 30 and a second die 40 for secondary drawing the wire rod 1 passed through the ball rolling rolls 20 and 30.

The first and second dies 10 and 40 are spaced apart by a predetermined distance, and the second die 40 has an inner diameter smaller than that of the first die 10. The wire rod passes through each die 10 and 40 by drawing force, and each diameter 10 and 40 passes through each die 10 and 40 to reduce the diameter according to the reduction rate. In the present embodiment, only two dies of the first and second dies 10 and 40 are illustrated, but at least one other die may be additionally disposed behind the dies.

The ball rolling rolls 20 and 30 press the wire rod 1 by a rotational operation to push the wire rod 1 forward to roll the wire rod 1. The ball rolling rolls 20 and 30 may include an elliptical ball rolling roll 20 and a circular ball rolling roll 30 disposed behind the ball rolling rolls 20 and 30.

The elliptical ball rolling roll 20 has an elliptical cross section through which the wire rod 1 passes, and a semi-elliptic groove 22 is formed on a pair of roll surfaces facing each other. In addition, the circular ball rolling roll 30 has a circular cross section through which the wire rod 1 passes, and a semicircular groove 32 is formed on a pair of roll surfaces facing each other.

The elliptical ball rolling rolls 20 and the circular ball rolling rolls 30 are arranged to be staggered from each other by 90 degrees. As in this embodiment, the elliptical ball rolling roll 20 may be in the form of upper and lower rolls, the circular ball rolling roll 30 may have the form of left and right rolls.

9 (a) shows a cross section of the wire rod 1 passing through the elliptical ball rolling roll 20. According to this, the cross-sectional shape formed by the elliptical ball rolling roll 20, that is, the upper and lower rolls is a flat oval. Therefore, as the wire rod 1 of circular cross section passes through the elliptical ball rolling roll 20, the height is reduced and the width is deformed.

And (b) of FIG. 9 has shown the cross section of the wire rod 1 which passes the circular ball rolling roll 30. As shown to FIG. According to this, the circular ball rolling roll 30, ie, the left and right rolls, has a circular cross section. Therefore, the wire rod 1 deformed into an elliptical cross-section after passing through the elliptical ball rolling roll 20 is deformed to increase in height and decrease in width as it passes through the circular ball rolling roll 30.

As described above, the provision of the eutectic rolling to the drawing process is a method for reducing the triaxial tensile stress, and the rolling and drawing can be distributed in parallel to the degree of damage. You can increase your reduction ability.

In more detail, the direction of plastic deformation due to drawing can be changed by imparting a rolling method. However, a plastically rolled material is strengthened in tensile strength due to a different strain path for the drawn material. A marked decrease in work hardening rate is observed. That is, the cumulative deformation amount can be increased by varying the plastic strain path by a combination of primary drawing, ball rolling and secondary drawing.

The effect of cold rolling in the above process is to recover the dislocations generated by the drawing process, that is, to promote dissipation of dislocations.

The recovery process of dislocations is caused by the dislocation tangle level being lowered due to the effect that the strain path is changed by the ball rolling, causing the oscillating movements of the dislocations to be fixed and the dislocations of the dislocations being loosened. . Depending on how the dislocations produced during plastic deformation are distributed, the work hardening rate may increase or decrease.

The reason why the rolling process is given to the wire rod 1 on which the primary drawing is completed is to give a strain path substantially perpendicular to the deformation direction by drawing. In drawing and ball rolling, the relationship between the deformation directions is largely close to vertical. The drawing process has a large amount of processing in the linear direction (x-axis), and the ball rolling has a large amount of strain in the y-axis and z-axis directions.

Ball rolling with a deformation direction different from the drawing deformation direction promotes recovery and disappearance for a significant amount of dislocations produced by the drawing process. This results in a significant reduction in the triaxial stress and work hardening rate at the center of the wire rod 1 during primary drawing, so that even if secondary drawing is applied, no central chevron cracks and micro share bands are produced, resulting in steel with a 50% or more cumulative cross-sectional reduction rate. Processing is possible.

FIG. 10 shows a comparison of the strain distribution of the cross section of the raw material after drawing and the strain distribution of the cross section of the raw material after ball rolling. Referring to this, the drawing process is characterized by a large amount of surface deformation compared to the center of the material, the ball rolling is characterized by a large amount of deformation of the center relative to the surface portion, it is possible to give a uniform plastic strain to the material surface and the center as a whole There are advantages to it.

Equivalent strain can be calculated by multiplying the sum of the squares of each of the strain components x, y, and z by multiplying 2/3 to obtain the square root, and subtracting the final cross-sectional area minus the final cross-sectional area. Cross section reduction rate.

On the other hand, in the process leading to the first drawing step, the ball rolling step, and the second drawing step, the wire rod 1 is drawn at a cross-sectional reduction rate in the range of 10 to 40% in the first drawing step, and 10 to 40 in the ball rolling step. Rolling is carried out at a section reduction rate in the range of% and drawn at a section reduction rate in the range of 10 to 40% in the secondary drawing step so that the cumulative section reduction rate of the wire rod 1 is finally processed to be 50% or more.

Machining so that the cumulative cross-sectional reduction rate of the wire rod 1 is 50% or more is, as described above, in order to secure fine austenite grains (5 μm or less) effective for improving corrosion resistance, which is directly above the austenite transformation point (Ac ₃ ). Heat treatment is required at Ac ₃ ~ Ac ₃ +80 ° C, and a significant amount of dislocation density (4.0x10 ⁸ mm ^-2 or more) is used as a means of promoting atom diffusion to secure austenite single phase at low heat treatment temperature. To create it. In addition, there should be no processing problems (disruption) in securing such dislocation density, and the present invention provides a steel wire manufacturing process (hereinafter referred to as a 'hybrid process') of "primary drawing → ball rolling → secondary drawing" for this purpose. To present.

The first die 10 used for primary drawing is configured such that the wire rod 1 is drawn at a rate of cross-sectional reduction in the range of 10-40%. If the cross-sectional reduction rate is less than 10%, it is difficult to obtain the proper dislocation density (4.0x10 ⁸ mm ^-2 or more) required for grain refinement, and if it exceeds 40%, the sebron crack occurs due to the increase of the triaxial stress in the center, This is because the micro band is generated to adversely affect the mechanical properties.

More specifically, the reduction rate of the cross section of the wire rod 1 in the primary drawing step is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, and 10 to 40% for 0.3 to 0.6% C medium carbon steel and its alloy steel. In the case of 30% range, 0.6-1.0% C high carbon steel and its alloy steel, it is preferable to have 10-40% range. It is because it is difficult to secure the proper dislocation density required for grain refinement below the lower limit of the cross-sectional reduction rate, and if it exceeds the upper limit, sebron crack occurs due to the increase of triaxial stress in the center, and micro shear band is generated due to instability of machining. Because it adversely affects.

The ball rolling rolls 20 and 30 are configured such that the wire rod 1 is rolled at a section reduction rate in the range of 10 to 40%. The reduction rate of the cross section of the ball rolling refers to the reduction rate of the cross section when the wire rod 1 finally passes through the elliptical ball rolling roll 20 and the circular ball rolling roll 30. If the cross-sectional reduction rate is less than 10%, it is difficult to insert the material into the circular ball rolling roll (30). In addition, at a cross-sectional reduction rate of 10%, the strain path is changed, so that the dislocations generated during the drawing process are recovered and the effect of promoting dissipation of the dislocations is insufficient. The effect of eliminating triaxial stress on the center and reducing the work hardening rate is insufficient.

When the cross-sectional reduction rate of the ball rolling exceeds 40%, the effect of removing the triaxial stress applied to the center of the wire rod 1 during primary drawing and the effect of reducing the work hardening rate are saturated.

More specifically, the reduction rate of the cross section of the wire rod 1 in the step of rolling is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, and 10 to 30 for 0.3 to 0.6% C medium carbon steel and its alloy steel. % Range, 0.6 ~ 1.0% C For high carbon steels and their alloy steels, it is desirable to have a range of 10-25%.

The second die 40 used in the secondary drawing step is configured such that the wire rod 1 is drawn at a rate of cross-sectional reduction in the range of 10-40%. If the cross-sectional reduction rate is less than 10%, it is difficult to obtain the proper dislocation density (4.0x10 ⁸ mm ^-2 or more) required for grain refining, and at 40% or more, Severon cracks occur due to the increase of the triaxial stress in the center, and the machining is unstable. This is because micro bands are generated, which adversely affects mechanical properties.

More specifically, the reduction rate of the cross section of the wire rod in the secondary drawing step is in the range of 10 to 40% for low carbon steel and its alloy steel of 0.3% C or less, and 10 to 30% for 0.3 to 0.6% C medium carbon steel and its alloy steel. In the case of 0.6 to 1.0% C high carbon steel and its alloy steel, it is preferable to have a range of 10 to 30%. It is difficult to obtain the proper dislocation density (4.0x10 ⁸ mm ^-2 or more) required for grain refinement below the lower limit of the cross-sectional reduction rate, and if it exceeds the upper limit, sebron cracks are generated due to the increase of triaxial stress in the center. This is because the micro-share band is generated, which adversely affects mechanical properties.

The wire rod processed through the hybrid machining process as described above preferably has a dislocation density of 4.0 × ^{10 8} mm ⁻² or more. When the dislocation density is less than 4.0x10 ⁸ mm ^-2 , the initial microstructure of the drawn material during austenite heat treatment in the conventional induction heating and atmospheric heating methods in the range of Ac ₃ to Ac ₃ + 80 ° C, which is directly above the austenite transformation point (Ac ₃ ) This is because ferrite + pearlite is not completely reused as austenite during heat treatment, and some remain and adversely affect the toughness of mechanical properties.

This is because it is difficult to increase the fraction of grain boundaries having a grain boundary angle of 15 ° or less, which is effective for corrosion resistance, even when grain refinement is made at a dislocation density of less than 4.0 × ^{10 8} mm ⁻² .

On the other hand, after the hybrid machining process as described above is completed austenite heat treatment of the wire rod (1). Such austenite heat treatment is preferably carried out in a temperature range of Ac ₃ ~ Ac ₃ +80 ℃ which is directly above the austenite transformation point (Ac ₃ ). The heat treatment below the austenite transformation point (Ac ₃ ) is because the cold working microstructure does not form austenite, and at a higher temperature than Ac ₃ +80 ℃, the grains rapidly grow to secure the austenite grains of 7 μm or less. Because you can't.

In addition, during the austenite heat treatment of the wire rod 1, it is preferable to limit the undissolved pearlite to have a fraction of 5% or less. This is because coarse cementite, which is a component of undissolved pearlite distributed in the heat treatment material, adversely affects toughness when the undissolved pearlite fraction exceeds 5%.

After the austenite heat treatment of the wire rod 1, it is preferable that the fraction of the grain boundaries having a grain boundary angle of 15 ° or less is 20% or more. This is because the effect of improving the corrosion resistance is insufficient when the fraction of the grain boundary having a grain boundary angle of 15 ° or less is less than 20%.

After the austenite heat treatment, the average size of the grain size is preferably 7 탆 or less. In order to improve the corrosion resistance, it is required to set the fraction of grain boundaries having grain boundaries of 15 ° or less to 20% or more, since it is difficult to secure a fraction of 20% or more when the austenite grain size is larger than 7 µm. .

EXAMPLE

In the following examples, AISI5120B is used as the low carbon alloy steel, SAE9254 is used as the medium carbon alloy steel, and AISI1080 is used as the high carbon alloy steel.

Hybrid processing used 17 ~ 20mmΦ wire for each condition, using primary drawing die, giving primary drawing with 10 ~ 40% cross-sectional reduction rate, and reducing the cross-sectional reduction rate under normal cold cold rolling condition 10 ~ 40% After the second drawing was carried out in the range of 10 to 40% reduction rate of the cross section under normal conditions.

Dislocation density measurement and evaluation after hybrid processing and normal drawing processing were evaluated by measuring the number of dislocations per unit area using a transmission electron microscope.

The central crack was observed using an optical microscope, and the direction of observation was determined by cutting the central part in the linear direction after hybrid processing, and the observation of the micro-share band was determined by observing with a scanning electron microscope.

Austenitic heat treatment used induction heating, the heating time was maintained after 5 seconds oil-cooled, tempering was carried out by the atmospheric heating method.

The grain size of austenite was measured by the KS standard (KS D 0205), and the tissue fraction of undissolved pearlite after heat treatment was evaluated by an image analyzer. In order to investigate the tensile properties and cross-sectional reduction rate for the materials prepared as described above, KS standard (KS B0801) tensile test piece was used and tested at a cross head speed of 5mm / min during the tensile test.

The grain boundary angles were evaluated using an EBSD (Electron Backscatter Diffraction) device that can measure the azimuth difference between the crystals. At this time, the grain boundary angle effective for improving corrosion resistance was 15 ° or less, and the grain fraction was evaluated on the basis of 15 ° based on the grain refinement.

In order to demonstrate the effect of improving the corrosion resistance through the steel wire manufacturing method according to the present invention was tested by a conventional evaluation method. In the case of the low carbon alloy steel AISI5120B, a plate-shaped test piece was prepared and evaluated by CCT (Cyclic Corrosion Test) method to measure the breaking time by salt spray after applying bending stress. At this time, the tensile strength was evaluated at a bending stress of 900MPa by Q / T heat treatment at 1500 ± 20 MPa level. In the case of SAE9254 of medium-carbon alloy steel, the actual spring was prepared and evaluated by the PV1210 method of measuring the breaking time by salt spray after applying the spring tightening stress. At this time, the tensile strength was evaluated at a spring tightening stress of 1300 ± 20 MPa by Q / T heat treatment at 2000 ± 20 MPa level. In the case of the high-carbon alloy steel AISI1080, a plate-shaped test piece was prepared and evaluated by CCT (Cyclic Corrosion Test) method to measure the breaking time by salt spray after applying bending stress. At this time, the tensile strength was evaluated at a bending stress of 1000MPa by Q / T heat treatment at a level of 1800 ± 20 MPa.

Brine spray proceeds to 5% brine spray (35 ℃, 4 hr, pH 6.5 ~ 7.2) and dried (humidity 50%, 23 ℃, 4 hr) to wet (humidity 100%, 40 ℃, 16 hr) to break The time was evaluated. The measurement and evaluation results are shown in Table 1 below.

Hybrid processing (primary drawing → ball rolling → secondary drawing) under the conditions shown in Table 1, and the core crack and micro shear in low carbon alloy steel (AISI5120B), medium carbon alloy steel (SAE9254) and high carbon alloy steel (AISI1080) products Bands and dislocation densities were measured, and the results are shown in Table 1 below.

1st drawing
* RA (%) Ball rolling
* RA (%) 2nd drawing
* RA (%) Total accumulation
* RA (%) center
crack Micro
Share Band Dislocation density
(Number / mm ² ) Remarks Example 1 37 10 14 51 Not Occurred Not Occurred 4.0 x ^{10 8} Low carbon
Alloy steel Example 2 13 38 30 62 Not Occurred Not Occurred 4.3 x ^{10 8} Example 3 37 34 33 72 Not Occurred Not Occurred 4.8 x ^{10 8} Example 4 13 20 30 51 Not Occurred Not Occurred 4.2 x ^{10 8} Medium carbon
Alloy steel Example 5 27 20 24 56 Not Occurred Not Occurred 4.4 x ^{10 8} Example 6 27 28 27 62 Not Occurred Not Occurred 4.6 x ^{10 8} Example 7 27 13 22 51 Not Occurred Not Occurred 4.4 x ^{10 8} High carbon
Alloy steel Example 8 27 20 24 56 Not Occurred Not Occurred 4.8 x ^{10 8} Example 9 13 20 30 51 Not Occurred Not Occurred 4.5 x ^{10 8} Comparative Example 1 46 15 17 62 Occur Occur 4.3 x ^{10 8} Low carbon
Alloy steel Comparative Example 2 11 44 11 56 Occur Occur 4.1 x ^{10 8} Comparative Example 3 22 25 52 72 Occur Occur 4.8 x ^{10 8} Comparative Example 4 35 23 11 56 Occur Occur 4.1 x ^{10 8} Medium carbon
Alloy steel Comparative Example 5 17 35 18 56 Occur Occur 4.4 x ^{10 8} Comparative Example 6 11 12 37 51 Occur Occur 4.3 x ^{10 8} Comparative Example 7 46 15 17 62 Occur Occur 4.7 x ^{10 8} High carbon
Alloy steel Comparative Example 8 22 28 12 51 Occur Occur 4.5 x ^{10 8} Comparative Example 9 11 12 37 51 Occur Occur 4.3 x ^{10 8}

After the hybrid processing of Examples 1-9, the central part crack which caused the disconnection of the wire rod 1 did not generate | occur | produce, and the micro share band which affects a fall of toughness was not formed. However, in Comparative Examples 1 to 9, it was confirmed that central cracks and micro share bands were generated.

On the other hand, the center crack, micro shear band, and dislocation density in the product were measured under the conditions that the amount of drawing processing by the conventional drawing method and the amount of steel processing higher than that were given, and the results are shown in Table 2.

Drawing throughput
* RA (%) center
crack Micro
Share Band Dislocation density
(Number / mm ² ) Remarks Comparative Example 10 55 Occur Occur 3.5 x ^{10 8} Low carbon
Alloy steel Comparative Example 11 50 Occur Occur 3.8 x ^{10 8} Medium carbon
Alloy steel Comparative Example 12 45 Occur Occur 3.3 x ^{10 8} High carbon
Alloy steel Comparative Example 13 20 Not Occurred Not Occurred 1.8 x ^{10 8} Low carbon
Alloy steel Comparative Example 14 20 Not Occurred Not Occurred 2.2 x ^{10 8} Medium carbon
Alloy steel Comparative Example 15 20 Not Occurred Not Occurred 2.8 x ^{10 8} High carbon
Alloy steel

As in Comparative Examples 10 to 12, the crack in the center portion, which causes breakage of the workpiece during steel drawing, was generated, and a micro shear band was formed to affect the toughness. In Comparative Examples 13 to 15, which are normal conditions, no center cracks and micro share bands were formed, but the dislocation density distributed in the material after drawing was found to be very low compared to Examples 1 to 9, and therefore, drawing under normal conditions was performed. It can be seen that it is not effective for grain refinement.

On the other hand, the processing was carried out under the hybrid processing conditions of Examples 1, 4, and 7 and the processing was carried out under normal drawing processing conditions such as Comparative Examples 13, 14, and 15 at the heating temperature Ac3 to Ac3 + 80 ° C. The heat treatment was performed to determine the unemployed perlite structure fraction, tensile section reduction rate, austenite average grain size, austenite grain boundary angle, and corrosion resistance, and the results are shown in Table 3 below.

Austenitic heat treatment * Ac ₃ + temperature (℃) Unemployed Perlite Fraction (%) Tensile Test Reduction Rate (%) Austenitic Average Grain Size (μm) Austenitic grain boundary less than 15 ° fraction (%) Corrosion Resistance Average Break Time (hr) Remarks Example 10 Ac ₃ +30 0 65 4 40 400 Example 1 Example 11 Ac ₃ +80 0 65 5 30 380 Example 12 Ac ₃ +30 0 45 4 50 240 Example 4 Example 13 Ac ₃ +80 0 45 5 40 220 Example 14 Ac ₃ +30 0 40 4 45 220 Example 7 Example 15 Ac ₃ +80 0 40 5 35 200 Comparative Example 16 Ac ₃ +30 5 55 8 15 120 Comparative Example 13 Comparative Example 17 Ac ₃ +80 3 50 10 10 115 Comparative Example 18 Ac ₃ +30 25 20 8 16 115 Comparative Example 14 Comparative Example 19 Ac ₃ +80 15 25 10 12 105 Comparative Example 20 Ac ₃ +30 30 20 8 14 105 Comparative Example 15 Comparative Example 21 Ac ₃ +80 20 30 10 11 100 Comparative Example 22 Ac ₃ +150 0 60 16 7 80 Comparative Example 13 Comparative Example 23 Ac ₃ +150 0 45 15 9 50 Comparative Example 14 Comparative Example 24 Ac ₃ +150 0 40 17 8 30 Comparative Example 15

* A _C3 + Temperature: A _C3 is a transformation point that becomes austenite single phase when heated, and it is heat-treated by setting A _C3 based on the following standard, which is different depending on the carbon content and the amount of alloying elements added.

* A _C3 = 910-203 (C) ^1/2 -15Ni + 44.5Si + 104V + 31.5Mo + 13.1W

As a result of the heat treatment (Examples 10 to 15 and Comparative Examples 16 to 21) performed at Ac3 to Ac3 + 80 ° C., which is a temperature condition for refining grains, in the case of Examples 10 to 15, unemployed pearlite does not remain. In the case of Comparative Examples 16 to 21, unemployed pearlite remained in the range of 3 to 30%. 35-50% for Examples 10-15 at grain boundaries having austenite grain boundaries of 15 ° or less. However, in the case of Comparative Examples 16 to 21 was found to show a large difference to 10 to 16% level.

Looking at the corrosion resistance evaluation results, it was found that the breakage occurs in 200 to 400 hours in Examples 10 to 15, but early breakage in 100 to 120 hours in Comparative Examples 16 to 21. From these results, it can be seen that the steel wires processed through the hybrid processing of the present invention have significantly improved corrosion resistance.

Looking at the results of Comparative Examples 22 to 24 in which the wire rod of the usual drawn amount was heat-treated at a normal heating temperature, the grain boundary fraction having an austenite grain size of 15 to 17 µm and a grain boundary angle of 15 ° or less was 7 to 9 It was shown as a% level, even in the corrosion resistance evaluation appeared to cause early breakage at 30 to 80 hours, it was shown to be very inferior to the case of Examples 10 to 15.

Although the foregoing has been described with reference to specific embodiments of the present invention, those skilled in the art may vary the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. It will be understood that modifications and changes can be made.

10: first die 20: oval ball rolling roll
22: oval groove 30: round ball rolling roll
32: oval groove 40: second die
1: wire rod

Claims (15)

Drawing the wire by passing the wire through the first die;
A ball rolling step of rolling the wire rod having passed through the first die through at least one ball rolling roll; And
A second drawing step of drawing the wire rod having passed through the ball rolling roll through a second die;
The wire is drawn at a cross-sectional reduction rate in the range of 10 to 40% in the primary drawing step, rolled at a cross-sectional reduction rate in the range of 10 to 40% in the step of rolling, and in the range of 10 to 40% in the secondary drawing step. A method for producing a steel wire, which is drawn at a section reduction rate and finally processed so that the cumulative section reduction rate of the wire rod is 50% or more. The method of claim 1, wherein the reduction rate of the cross section of the wire rod in the first drawing step,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In the case of high carbon steel and its alloy steel, the steel wire manufacturing method characterized in that it has a range of 10-40%. The method of claim 1, wherein the reduction rate of the cross section of the wire rod in the step of rolling the ball,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In the case of high carbon steel and its alloy steel, the steel wire manufacturing method characterized in that it has a range of 10-25%. According to claim 1, The reduction rate of the cross section of the wire rod in the secondary drawing step,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In the case of high carbon steel and its alloy steel, the steel wire manufacturing method characterized in that it has a range of 10-30%. The method of claim 1,
The wire rod, which has completed the secondary drawing step, has a dislocation density of 4.0 × ^{10 8} mm ⁻² or more. The method of claim 1,
After the completion of the secondary drawing step the wire rod manufacturing method characterized in that it further comprises the step of austenitic heat treatment. The method of claim 6,
The austenitic heat treatment is a method for producing a steel wire, characterized in that carried out in the temperature range of Ac ₃ ~ Ac ₃ +80 ℃ directly above the austenite transformation point (Ac ₃ ). The method of claim 6,
The method for producing a steel wire, characterized in that the undissolved pearlite has a fraction of 5% or less during the austenite heat treatment. The method of claim 6,
After the austenitic heat treatment, the fraction of the grain boundary having a grain boundary of 15 ° or less is 20% or more. The method of claim 6,
After the austenitic heat treatment, the average size of the grain size is 7㎛ or less method for producing a steel wire. A first die for primary drawing the wire rod;
One or more ball rolling rolls for ball rolling the wire rod passed through the first die; And
A second die for secondary drawing the wire rod having passed through the ball rolling roll;
The first die is configured such that the wire is drawn at a cross sectional reduction rate in the range of 10 to 40%, the ball rolling roll is configured such that the wire is rolled at a cross sectional reduction rate in the range of 10 to 40%, and the second die is The wire is configured to draw at a rate of cross section reduction in the range of 10-40%,
And said first and second dies and said ball rolling roll are configured so that the cumulative reduction ratio of said wire rod is 50% or more. The method according to claim 11, wherein the ball rolling roll,
An elliptical ball rolling roll having an elliptical cross section through which the wire rod passes; And
The steel wire manufacturing apparatus, characterized in that it comprises a circular ball rolling roll is installed in the rear of the elliptical ball rolling roll, the cross section through which the wire passes. The method of claim 11, wherein the first die,
The reduction rate of the cross section of the wire rod,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In case of high carbon steel and its alloy steel, steel wire manufacturing apparatus characterized in that it is configured to have a range of 10 to 40%. The method according to claim 11, wherein the ball rolling roll,
The reduction rate of the cross section of the wire rod,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In the case of high carbon steel and its alloy steel, the apparatus for producing a steel wire, characterized in that configured to have a range of 10 to 25%. The method of claim 11, wherein the second die,
The reduction rate of the cross section of the wire rod,
10% to 40% for low carbon steels and alloy steels below 0.3% C,
10% to 30% for 0.3 to 0.6% C medium carbon steels and their alloy steels,
0.6 ~ 1.0% C In the case of high carbon steel and its alloy steel, the apparatus for producing steel wire, characterized in that configured to have a range of 10 to 30%.

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