JP2023508022A - Bearing wire rod and manufacturing method thereof - Google Patents

Abstract

The present invention provides a wire rod for bearings and a method of manufacturing the same, which can shorten or omit the softening heat treatment required in the cold working of automobiles, construction parts, and the like.
[Means for Solving Problem] In terms of % by weight, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 1.0 to 2.0 %, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the rest consists of Fe and inevitable impurities, and the prior austenite grain size of the microstructure is 3 to 10 μm and the sum of the lengths of high-angle grain boundaries having a misorientation angle of 15° or more is 1,000 to 4,000 mm/mm ² per unit area.
[Selection drawing] Fig. 5

Description

TECHNICAL FIELD The present invention relates to a wire rod for bearings and a manufacturing method thereof.

As the carbon content of the wire increases, the strength of the material increases sharply, making direct forming and processing difficult. Proeutectoid cementite, which is precipitated at the grain boundaries of the former austenite during cooling, drastically reduces the softness or toughness of the material. descend.
A spheroidizing heat treatment is generally performed to soften the wire. The spheroidizing heat treatment induces a homogeneous particle distribution by spheroidizing the cementite to improve cold workability during cold forming. It also lowers the hardness of the material being machined in order to improve the life of the working dies.
On the other hand, wire for cold heading (CHQ) is drawn first to accelerate spheroidization, but wire for bearings with a relatively high carbon content is drawn first. In this case, disconnection due to internal defects may occur.

Generally, one or more softening heat treatments are performed in order to manufacture a wire rod for bearing steel from a steel wire. Thereafter, wire drawing and heat treatment processes are additionally performed to improve the cold forgeability, and the cold forgeability is ensured by the tensile strength and spheroidization rate after the softening heat treatment.
However, in order to soften the wire for bearings, long-term treatment at a high temperature of 700 to 800°C for 30 hours or more is required, so a lot of heat treatment costs and production time are required, increasing the manufacturing cost of the product. cause it to Therefore, there is a demand for development of a bearing wire and a method of manufacturing the same that can shorten or omit the additional softening heat treatment process.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a bearing wire and a method of manufacturing the same, which can shorten or omit the softening heat treatment required in the cold working of automobiles, construction parts, and the like.

The wire rod for bearings of the present invention has, in weight percent, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, and Cr: 1.0. ~2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the rest consists of Fe and inevitable impurities, and the prior austenite grain size of the microstructure is 3 to 10 μm, and the sum of the lengths of high-angle grain boundaries having a misorientation angle of 15° or more is 1,000 to 4,000 mm/mm ² per unit area.

The sum of the lengths of low-angle grain boundaries with a misorientation angle of 15° or less is 250 to 800 mm/mm ² per unit area, and the proportion of the grain boundaries with a misorientation angle of 5° or less among the low-angle grain boundaries is preferably 40 to 80%.
It is preferable that the fine structure is composed of reticular proeutectoid cementite at grain boundaries and pearlite within grains.
The lamellar spacing within the perlite can be 0.05-0.2 μm.

The tensile strength is preferably 1,200 MPa or more, and the cross-sectional area reduction rate (RA) is preferably 20% or more.
The average aspect ratio of cementite after the first softening heat treatment is preferably 2.5 or less.
Tensile strength after one softening heat treatment may be 750 MPa or less.

In the method for producing a wire rod for bearings of the present invention, in weight %, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the rest is Fe and unavoidable impurities. heating in a temperature range of Ae1 to Acm ° C., finishing hot rolling at a deformation amount equal to or greater than the critical deformation amount expressed by the following formula (1) to produce a wire rod; cooling to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and then cooling at a rate of 1° C./sec or less.
Formula (1): −1.6 Ceq ² +3.11 Ceq −0.48
Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean weight percent of each element.

The wire can satisfy the following formula (2).
Formula (2): Tpf-Tf≤50°C
Here, Tpf is the average surface temperature of the wire before finish hot rolling, and Tf is the average surface temperature of the wire after finish hot rolling.
The heating time is preferably 90 minutes or less.

The average austenite grain size (AGS) before finish hot rolling is preferably 5 to 20 μm.
After cooling, a softening heat treatment step may be further included in which the wire is heated at Ae1 to Ae1+40° C. and maintained for 5 to 8 hours.
After the softening heat treatment, cooling to 660° C. at a rate of 20° C./hr or less may be further included.

According to the present invention, the bearing wire and the method for manufacturing the same according to the embodiments of the present invention can shorten or omit the softening heat treatment time, thereby reducing the cost of the manufacturing process.

1 is a microstructure photograph of the wire of Comparative Example 1 of the present invention taken with an optical microscope (OM) before finishing hot rolling. 1 is a microstructure photograph of the wire rod of Example 1 of the present invention taken with an optical microscope (OM) before finish hot rolling. 1 is a microstructure photograph taken with a scanning electron microscope (SEM) after finish hot rolling and cooling of the wire of Comparative Example 1 of the present invention; 1 is a microstructure photograph taken with a scanning electron microscope (SEM) after finish hot rolling and cooling of the wire rod of Example 1 of the present invention; FIG. 1 is a photograph of grain boundary characteristics observed through SEM-EBSD after finish hot rolling and cooling of the wire rod of Comparative Example 1 of the present invention; FIG. FIG. 1 is a photograph of grain boundary characteristics observed through SEM-EBSD after finish hot rolling and cooling of the wire rod of Example 1 of the present invention; FIG. 1 is a microstructure photograph taken with a scanning electron microscope (SEM) after the wire rod of Comparative Example 1 of the present invention was subjected to spheroidizing heat treatment; 1 is a microstructure photograph taken with a scanning electron microscope (SEM) after the wire rod of Example 1 of the present invention was spheroidized by heat treatment;

The bearing wire according to one embodiment of the present invention contains, in weight %, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr : 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the rest consists of Fe and unavoidable impurities, prior austenite crystals of fine structure The grain size is 3 to 10 μm, and the sum of the lengths of high-angle grain boundaries with a misorientation angle of 15° or more is 1,000 to 4,000 mm/mm ² per unit area.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to fully convey the spirit of the invention to those of ordinary skill in the art to which the invention pertains. The invention is not limited to the examples presented here, but can be embodied in other forms. In the drawings, parts irrelevant to the description are omitted in order to clarify the present invention, and the sizes of constituent elements are exaggerated to facilitate understanding.
Throughout the specification, when a part is referred to as "comprising" a component, this does not exclude other components, and may further include other components, unless specifically stated to the contrary. means
Singular references include plural references unless the context clearly dictates otherwise.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.
Bearing wire rods are sometimes subjected to a spheroidizing heat treatment to ensure workability. The spheroidizing heat treatment is an additional process that requires a lot of heat treatment cost and time, thus increasing the manufacturing cost.
The inventors of the present invention have extensively researched measures for shortening or omitting the spheroidizing softening heat treatment in the production of a wire rod for bearings. As a result, by optimizing the alloy composition and manufacturing conditions and deriving the characteristics of the grain boundaries, it was confirmed that the softening heat treatment time can be shortened or omitted, and the present invention was completed.

The bearing wire according to one embodiment of the present invention contains, in weight %, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr : 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), and the rest consists of Fe and unavoidable impurities.
The role and content of each component contained in the bearing wire according to the present invention are described below. % for the following components means % by weight.

The content of C is 0.8-1.2%.
C (carbon) is an element added to ensure the strength of the product. If the C content is less than 0.8%, the strength of the base material is reduced, making it difficult to secure sufficient strength after the softening heat treatment and the forging process followed by the quenching and tempering heat treatments. On the other hand, if the content is excessive, new precipitates such as _M7C3 may be formed and center segregation may occur during solidification _of blooms or billets. %. Preferably, the content of C is 0.8-1.1%.

The Si content is 0.01-0.6%.
Si (silicon) is a representative substitutional element, and is an element advantageous for ensuring strength through solid-solution strengthening. If the Si content is less than 0.01%, it is difficult to ensure the strength and hardenability of the wire. On the other hand, if the content is excessive, the strength may increase during forging after the softening heat treatment, making it difficult to ensure cold forgeability, so the upper limit is limited to 0.6%.

The content of Mn is 0.1-0.6%.
Mn (manganese) is an element that strengthens the solid solution by forming a substitutional solid solution in the matrix structure, and is an austenite-forming element that is added to ensure the target strength without lowering the softness. When the content of Mn is less than 0.1%, it is difficult to ensure strength and toughness due to solid-solution strengthening of the wire. On the other hand, if the content of Mn, which is an austenite-forming element, is excessive, the cold Acm transformation point is lowered during forging after the softening heat treatment, and center segregation may occur, resulting in an uneven wire structure. The upper limit is limited to 0.6%.

The Cr content is 1.0-2.0%.
Cr (chromium), like Mn, is an element advantageous for improving the hardenability of the wire rod and securing the martensite structure. If the Cr content is less than 1.0%, it is difficult to obtain a fine martensite structure during the quenching and tempering heat treatments performed after the softening heat treatment and forging processes. On the other hand, if the content is excessive, center segregation may occur and a large amount of low-temperature structure may be formed in the wire, so the upper limit is limited to 2.0%.

The Al content is 0.01-0.06%.
Aluminum (Al) not only has a deoxidizing effect, but also precipitates Al-based carbonitrides to suppress the growth of austenite crystal grains. Added. On the other hand, if the content is excessive, the generation of hard inclusions such as Al ₂ O ₃ increases, and there is a risk that the inclusions may clog nozzles during continuous casting, so the upper limit is set to 0.06%. limit.

The content of N is 0.02% or less (0 is excluded).
Nitrogen (N) has a solid-solution strengthening effect, but if its content is excessive, there is a risk that the toughness and softness of the material will be inferior due to solid-solution nitrogen that is not combined with nitrides, so in the present invention, it is controlled as an impurity. , limiting its upper limit to 0.02%.

The remaining component of the present invention is iron (Fe). However, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in during normal manufacturing processes, and this cannot be ruled out. Examples of unavoidable impurities include P (phosphorus) and S (sulfur). These impurities are known to anyone who is skilled in the normal manufacturing process, and therefore the full content thereof will not be specifically referred to in this specification.
On the other hand, in the microstructure of the bearing wire according to one embodiment of the present invention, due to prior austenite crystal grains, network type proeutectoid cementite exists at grain boundaries and complete pearlite exists within grains.
Also, according to an embodiment of the present invention, the size of the prior austenite grains in the microstructure is 3-10 μm.

During the softening heat treatment, the cementite in the pearlite structure changes its shape from plate-like to spherical, and the strength of the wire gradually decreases according to the degree of progress of the spherical formation.
During the softening heat treatment, the metal atoms move to various diffusion paths through the defect space in the material. ) and grain boundaries. Since dislocations and grain boundaries have relatively large spaces compared to atomic defects, they can diffuse at high speeds.
On the other hand, the heat treatment time for the softening heat treatment is determined by the diffusion rate of each atom, and the most important factor controlling such diffusion rate is the grain boundary.

In the present invention, high-angle grain boundaries and low-angle grain boundaries are separated by misorientation between grains with grain boundaries interposed therebetween in the grain boundary structure, and the respective distributions are controlled. bottom. Specifically, the mutual relationship between adjacent grains was quantified by a misorientation angle value, and 15° was used as a reference to classify high-angle grain boundaries of 15° or more and low-angle grain boundaries of 15° or less. The distribution of each crystal grain specified in the present invention applies not only to the surface layer portion of the wire but also to the entire region up to the central portion.
In order to effectively shorten the softening heat treatment time, it is ideal to secure a large number of high-angle grain boundaries by maximally refining the crystal grains and increasing the relative grain boundary area. In order to refine the grains, there arises a problem that the rolling load is increased, the equipment life is shortened, and the productivity is lowered.

Therefore, in the present invention, an attempt was made to control the size of the prior austenite crystal grains and to control the total length per unit area of the high-angle grain boundaries having a misorientation angle of 15° or more. Specifically, the prior austenite grain size (AGS) of the wire rod for bearings according to the disclosed embodiments is 3 to 10 μm, and the unit is the sum of the lengths of high-angle grain boundaries with a misorientation angle of 15° or more. 1,000 to 4,000 mm/mm ² per area.
On the other hand, the low-angle grain boundaries distributed in the high-angle grain boundaries and having a misorientation angle of 15° or less are places where dislocations generated by deformation during hot rolling gather, and the behavior of spheroidization during the softening heat treatment. can help improve cold forgeability. In the present invention, the sum of the lengths of low-angle grain boundaries having a misorientation angle of 15° or less is 250 to 800 mm/mm ² per unit area.

When the low-angle grain boundary length distribution is less than 250 mm/mm ² , the effect of shortening the softening heat treatment time is low, and when the low-angle grain boundary length distribution exceeds 800 mm/mm ² , As the dislocation density increases during rolling, there is a problem that the dislocation density is reduced due to partial recrystallization, or the grain size is non-uniform and develops into a bimodal form with different sizes.
On the other hand, the smaller the misorientation angle, the more dislocations are included. In the present invention, the proportion of grain boundaries with a misorientation angle of 5° or less among the low-angle grain boundaries is 40 to 80%. be.

Next, another aspect of the present invention, a method of manufacturing a wire rod for bearings, will be described in detail.
The wire of the present invention can be manufactured by manufacturing a billet having the above alloy composition and then reheating, rolling, and cooling the billet in multiple stages.

Specifically, in another aspect of the present invention, there is provided a method for manufacturing a wire rod for bearings, in terms of % by weight, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.5%. 1 to 0.6%, Cr: 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (excluding 0), the rest from Fe and unavoidable impurities In the step of heating the billet in the temperature range of 950 to 1,050 ° C., in the temperature range of Ae1 to Acm ° C., the wire rod is finished hot rolled with a deformation amount equal to or greater than the critical deformation amount expressed by the following formula (1). and cooling the wire to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and then cooling at a rate of 1° C./sec or less.
Formula (1): −1.6 Ceq ² +3.11 Ceq −0.48
Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean weight percent of each element.
The reason for limiting the numerical value of the content of alloying elements is as described above.

First, the present invention goes through a step of heating a billet having the above components in a temperature range of 950-1,050°C.
If the heating temperature is less than 950° C., the load applied to the rolling rolls increases, which may shorten the roll replacement period. On the other hand, if the heating temperature exceeds 1,050° C., rapid cooling is required for rolling, which makes it difficult to control the cooling and may cause cracks, etc., making it impossible to ensure good product quality. be.
Moreover, the heating is preferably performed within 90 minutes. If the heating is performed for more than 90 minutes, the depth of the decarburized layer on the surface of the wire may increase, and the decarburized layer may remain after the rolling is finished.

The heated billet is subjected to hot rolling in order of rough rolling/intermediate rough rolling/finish rolling and finish rolling to produce a wire rod. The hot rolling is preferably caliber rolling so that the billet has the shape of a wire rod. Finish hot rolling is performed with a deformation amount equal to or greater than the amount of deformation to produce a wire rod.
Since the rolling speed is very high during wire production, it corresponds to the dynamic recrystallization region. In the dynamic recrystallization regime, the austenite grain size (AGS) depends only on deformation rate and deformation temperature. The present invention is characterized by refining grains through dynamic recrystallization that occurs during rolling, and then maintaining the fine grains obtained during rolling until room temperature through rapid cooling.

In order to refine the grains during the final finish rolling, the interpass time between the rolls is controlled within 1 minute, and the austenite grain size (AGS) just before the finish rolling is in the range of 5 to 20 μm. After that, it is preferable to control the finish rolling temperature to Ae1 to Acm° C. during finish rolling.
If the temperature during the finish hot rolling is less than Ae1 ° C., the rolling load may increase and the equipment life may be shortened. The maintenance time until the end of the transformation becomes longer, and there is a possibility that the grain refinement effect, which is the aim of the present invention, may be greatly reduced.

In addition, the amount of deformation during hot rolling can be controlled within the above temperature range to be equal to or greater than the critical amount of deformation expressed by the following formula (1).
Formula (1): −1.6 Ceq ² +3.11 Ceq−0.48
Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean weight percent of each element.

The present inventors have derived the critical deformation expressed by Equation (1) in consideration of the correlation between Ceq and the deformation.
The amount of deformation is defined as −ln(1−RA), where RA is the reduction rate (RA<1) due to the rolling pass. If the amount of deformation does not reach the critical amount of deformation, the reduction amount is not sufficient, so it is difficult to sufficiently refine the crystal grains at the center of the wire, which adversely affects the spheroidization behavior of the wire during the softening heat treatment. effect.

On the other hand, the wire during hot rolling satisfies the following formula (2).
Formula (2): Tpf-Tf≤50°C
Here, Tpf is the average surface temperature of the wire before finish hot rolling, and Tf is the average surface temperature of the wire after finish hot rolling.

If the Tpf−Tf value exceeds 50° C., the deviation of the fine structure of the wire material becomes so large that a uniform fine structure cannot be secured, and supercooling occurs on the surface of the wire material, resulting in hard phases or crystal grains. may become coarse.
After hot rolling in the above temperature range, it is cooled to a temperature range of 500 to 600° C. at a speed of 3° C./sec or more, and then cooled at a speed of 1° C./sec or less to obtain the wire rod for bearings of the present invention. can be manufactured.

The cooling step is an essential step for ensuring the distribution of fine crystal grains, and in the present invention, it is possible to shorten the heat treatment time by controlling the cooling end temperature and cooling rate to accelerate diffusion. This is intended to ensure a fine structure.
If the cooling rate to the temperature range of 500 to 600° C. is less than 3° C./sec, it is difficult to maintain the fine crystal grains obtained through hot rolling below the transformation point, and the misorientation angle is 15° or less. The fraction of low-angle grain boundaries may be greatly reduced. On the other hand, if the cooling rate after reaching the temperature range of 500 to 600° C. exceeds 1° C./sec, a low-temperature structure such as bainite occurs, and softening does not proceed sufficiently despite the spheroidizing heat treatment. There is fear.
Next, the method may further include a step of coiling the wire that has undergone the cooling step and then subjecting it to a softening heat treatment.

For the softening heat treatment process, various heat treatment patterns can be applied according to the degree of softening required at a temperature of about Ae 1°C of the wire. In the present invention, after cooling, a softening heat treatment was performed by heating the wire to Ae1 to Ae1+40° C. and maintaining it for 5 to 8 hours.
When the heating temperature is less than Ae 1°C, there is a problem that the softening heat treatment time becomes long. On the other hand, if the heating temperature exceeds Ae1+40° C., the amount of spheroidized carbide seeds is reduced, and a sufficient softening heat treatment effect cannot be obtained. Further, the heating is preferably performed for 5 to 8 hours. When heating for more than 8 hours, there is a problem that the manufacturing process costs increase. On the other hand, if the heating is performed for less than 5 hours, the heat treatment may not proceed sufficiently, resulting in a problem that the aspect ratio of the cementite increases.

After the softening heat treatment step, a step of cooling to 660° C. at a rate of 20° C./hr or less is performed. At this time, if the cooling rate exceeds 20° C./hr, pearlite may be reformed due to the excessive cooling rate.
After the softening heat treatment, the tensile strength of the wire is preferably 750 MPa or less, and the average aspect ratio of cementite in the wire is preferably 2.5 or less. Specifically, 80% or more of carbide having a cementite average aspect ratio of 2.5 or less can be secured not only in the surface layer portion of the wire but also in the entire region up to the center portion.

In the present invention, the tensile strength of the wire rod can be controlled to a low level of 740 MPa or less with only one softening heat treatment, so cold heading or cold forging for manufacturing the final product is easy. As a result, it is possible to shorten or omit the time for the spheroidizing heat treatment, which is an additional process after manufacturing the wire, so that the cost can be reduced.
Hereinafter, the present invention will be described in more detail through examples. On the other hand, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.
Example

A steel material having a composition shown in Table 1 below was cast to produce a billet, and then hot rolled and cooled under the conditions shown in Table 2 below to produce a wire rod having a diameter of 10 mm. In Table 2, the average austenite grain size (Austenite Grain Size, hereinafter "AGS") before finish rolling was measured through a crop performed before finish hot rolling. Tpf is the average surface temperature of the wire before finish rolling, and Tf is the average surface temperature of the wire after finish rolling.

After that, the microstructure, characteristics of grain boundaries, and mechanical properties (tensile strength, area reduction ratio) of each of the manufactured examples and comparative examples were measured, and the results are shown in Table 3 below.
The tensile strength was measured by processing a hot-rolled wire rod into a tensile test piece according to ASTM E8 standard, manufacturing the steel wire according to the above steel wire manufacturing method, and performing a tensile test.
RA means a reduction ratio, which is obtained by measuring a change in the cross-sectional area of a tensile test piece broken during a tensile test of the material, and expresses the flexibility of the material numerically.

Average grain size (AGS) was measured using the ASTM E112 method. After hot rolling to produce a wire rod, the non-water-cooled part was removed and the test piece was sampled. Shown as an average value.
Grain boundary characteristics were obtained by taking a specimen by the same method as the grain size (AGS) measurement method, and then using SEM-EBSD to measure the surface, 1/4 point from the diameter, and 1/2 point from the diameter at x 700. An area of 130×130 μm ² was measured with a step-size of 0.1 μm, and the average value was shown. The average value of Confidence Index was 0.57 or more.

On the other hand, the wires of each of the examples and comparative examples were subjected to a spheroidizing heat treatment once under the conditions shown in Table 4 below, and then the average aspect ratio and tensile strength of cementite were measured, and the results are shown in Table 4 below. At this time, the spheroidizing heat treatment was performed on the test piece of the manufactured wire without first softening treatment and first wire drawing process, and the presence or absence of spheroidization was determined.
At this time, the cementite average aspect ratio of the wire after the spheroidizing heat treatment was obtained by photographing three fields of view with a 3000x SEM in a region of 1/4 to 1/2 in the diameter direction of the wire, and using an image measurement program to determine the cementite within the field of view. Statistically processed and displayed after automatic measurement of long axis/shortening.
Judgment of the presence or absence of spheroidization was performed by randomly photographing 10 or more locations using an SEM electron microscope, and then observing all carbides in a field of view of × 5,000. It was judged that spheroidization was performed when the occupancy rate of carbide was 80% or more.

Comparative Examples 1 to 4 are examples in which the alloy composition satisfies the standard specified in the present invention, but the following manufacturing process conditions deviate from the present invention, and are indicated as comparative examples.

1 and 2 are microstructure photographs taken with an optical microscope (OM) before finish hot rolling of the wires of Example 1 and Comparative Example 1 of the present invention, respectively, and FIGS. 1 and 2 are microstructure photographs taken with a scanning electron microscope (SEM) after finish hot rolling and cooling of the wire rods of Example 1 of the present invention and Comparative Example 1, respectively.
As shown in FIGS. 1 to 4, in Example 1, the prior austenite grain size (AGS) before finish hot rolling is relatively finer than that in Comparative Example 1, whereby the finish hot rolling And it can be confirmed that the crystal grains are fine even after cooling.

As shown in Table 3, the wires of Examples 1 to 3, which satisfy the alloy compositions and manufacturing conditions proposed by the present invention, have a prior austenite grain size (AGS) of 3 to 10 μm and a misorientation angle of ) is 15° or more, the length distribution of the high-angle grain boundaries is 1,000 to 4,000 mm/mm ² , and fine crystal grains can be secured. Moreover, the wires of Examples 1 to 3 ensured a high tensile strength of 1,200 MPa or more and had a cross-sectional area reduction rate of 20% or more as compared with the comparative example.

5 and 6 are photographs of grain boundary characteristics observed through SEM-EBSD after finish hot rolling and cooling of the wire rods of Example 1 and Comparative Example 1 of the present invention, respectively.
As shown in FIGS. 5 and 6, in Example 1, compared to Comparative Example 1, the degree of distribution of low-angle grain boundaries with a misorientation angle of 15° or less, which is shown in green and red. can be confirmed to be high.
As shown in Table 4, the wires of Examples 1 to 3, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, not only have a low tensile strength of 740 MPa or less after the first softening heat treatment, By securing fine crystal grains, spheroidized cementite having an average aspect ratio of 2.5 or less could be secured even with a shorter spheroidizing heat treatment than the conventional heat treatment of 30 hours or longer.

7 and 8 are microstructure photographs taken with a scanning electron microscope (SEM) after subjecting the wires of Example 1 and Comparative Example 1 of the present invention to a spheroidizing heat treatment, respectively.
As shown in FIGS. 7 and 8, in Example 1, spheroidal cementite was uniformly distributed compared to Comparative Example 1, so it can be confirmed that spheroidization was performed at a high speed.
In the case of Comparative Example 1, the Mn content was excessive and the crystal grains were not sufficiently refined during rolling as the Acm transformation point increased. As a result, even after the softening heat treatment, the cementite average aspect ratio was 8.5, and a spheroidized structure could not be obtained, and the tensile strength value was as high as 820 MPa.

In the case of Comparative Example 2, the finish hot rolling temperature exceeded 850° C., which is the temperature above the Acm° C. transformation point, and the cooling time required to complete the phase transformation was lengthened, so that the grain refinement effect was greatly reduced. . As a result, even after the softening heat treatment, the cementite average aspect ratio was 6.2, a spheroidized structure could not be obtained, and the tensile strength value was as high as 790 MPa.
In the case of Comparative Example 3, the composition range specified by the present invention was satisfied, but the Tpf-Tf value greatly exceeded 85°C and 50°C, and the temperature deviation inside/outside the material increased greatly during rolling, and the center part A coarse microstructure with an average grain size of 15 μm was formed. As a result, even after the softening heat treatment, the cementite average aspect ratio was 7.5, and a spheroidized structure could not be obtained, and the tensile strength value was as high as 810 MPa.

In the case of Comparative Example 4, the composition range specified by the present invention is satisfied, but the deformation amount is 0.32, which is far short of the critical deformation amount of 0.69, so a sufficient rolling reduction amount cannot be secured. Sufficient grain refinement was not achieved. As a result, even after the softening heat treatment, the cementite average aspect ratio was 5.5, and a spheroidized structure could not be obtained, and the tensile strength value was as high as 770 MPa.
As described above, according to the embodiments of the present invention, the fine grain distribution was generated by controlling the alloy composition and the manufacturing method. Accordingly, it is possible to shorten or omit the spheroidizing heat treatment process for softening the wire after manufacture, thereby ensuring the price competitiveness of the product.

While exemplary embodiments of the present invention have been described above, the present invention is not so limited and any person of ordinary skill in the art will appreciate the concept and scope of the following claims. It should be understood that various modifications and variations are possible without departing from the scope of the invention.

The bearing wire and the method for manufacturing the same according to the present invention can shorten or omit the softening heat treatment time, so that the manufacturing process cost can be reduced.

Claims (13)

% by weight, C: 0.8-1.2%, Si: 0.01-0.6%, Mn: 0.1-0.6%, Cr: 1.0-2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the rest consists of Fe and inevitable impurities,
The prior austenite grain size of the microstructure is 3 to 10 μm,
A wire rod for bearings, wherein the sum of lengths of high-angle grain boundaries having a misorientation angle of 15° or more is 1,000 to 4,000 mm/mm ² per unit area. The sum of the lengths of the low-angle grain boundaries having a misorientation angle of 15° or less is 250 to 800 mm/mm ² per unit area, and the grain boundaries having a misorientation angle of 5° or less among the low-angle grain boundaries The wire rod for bearing according to claim 1, wherein the ratio is 40 to 80%. 2. The wire rod for bearing according to claim 1, wherein the microstructure is composed of network-type proeutectoid cementite at grain boundaries and pearlite within grains. 4. The wire rod for bearing according to claim 3, wherein the layer spacing in pearlite is 0.05 to 0.2 μm. 2. The wire rod for bearings according to claim 1, having a tensile strength of 1,200 MPa or more and a cross-sectional area reduction rate (RA) of 20% or more. 2. The wire rod for bearings according to claim 1, wherein the average aspect ratio of cementite after the first softening heat treatment is 2.5 or less. 2. The wire rod for bearings according to claim 1, wherein the tensile strength after one softening heat treatment is 750 MPa or less. % by weight, C: 0.8-1.2%, Si: 0.01-0.6%, Mn: 0.1-0.6%, Cr: 1.0-2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the balance is Fe and inevitable impurities Heating the billet in the temperature range of 950 to 1,050 ° C.,
A step of finish hot rolling to produce a wire rod in a temperature range of Ae1 to Acm ° C. with a deformation amount equal to or greater than the critical deformation amount expressed by the following formula (1); A method of manufacturing a wire rod for bearings, comprising cooling to a temperature range of 500 to 600° C. and then cooling at a rate of 1° C./sec or less.
Formula (1): −1.6 Ceq ² +3.11 Ceq−0.48
(Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean weight percent of each element.) 9. The method of manufacturing a bearing wire according to claim 8, wherein the wire satisfies the following formula (2).
Formula (2): Tpf-Tf≤50°C
(Here, Tpf is the average surface temperature of the wire before finish hot rolling, and Tf is the average surface temperature of the wire after finish hot rolling.) 9. The method of manufacturing a bearing wire according to claim 8, wherein the heating time is 90 minutes or less. 9. The method for producing a bearing wire according to claim 8, wherein the austenite grain average size (AGS) before finish hot rolling is 5 to 20 μm. 9. The method of manufacturing a bearing wire according to claim 8, further comprising a softening heat treatment step of heating the wire at Ae1 to Ae1+40° C. and maintaining the temperature for 5 to 8 hours after cooling. 13. The method of claim 12, further comprising cooling to 660[deg.] C. at a rate of 20[deg.] C./hr or less after the softening heat treatment.

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