JP7389909B2 - Bearing wire rod and its manufacturing method - Google Patents

Description

The present invention relates to a bearing wire and a method for manufacturing the same.

As the carbon content of the wire increases, the strength of the material increases rapidly, making direct forming and processing difficult, and the softness or toughness of the material rapidly decreases due to pro-eutectoid cementite precipitated by prior austenite grain boundaries during cooling. descend.
Generally, a spheroidizing heat treatment is performed to soften the wire. Spheroidizing heat treatment spheroidizes cementite to improve cold workability during cold forming and induces homogeneous particle distribution. It also reduces the hardness of the material being processed to improve the life of the processing die.
On the other hand, wire rods for cold heading (CHQ) are first subjected to wire drawing to accelerate spheroidization, but wire rods for bearings, which have a relatively high carbon content, are subjected to wire drawing first. In this case, there is a risk of wire breakage due to internal defects.

Normally, in order to manufacture bearing steel wire from steel wire, it is subjected to softening heat treatment one or more times. Thereafter, wire drawing and heat treatment steps are additionally performed to improve cold forgeability, and cold forgeability is ensured by tensile strength and spheroidization rate after softening heat treatment.
However, in order to soften bearing wire rods, long-term treatment of 30 hours or more is required at high temperatures of 700 to 800 degrees Celsius, which requires a lot of heat treatment cost and production time, which increases the manufacturing cost of the product. cause it to happen. Therefore, there is a need to develop a bearing wire and a method for manufacturing the same that can shorten or omit an additional softening heat treatment process.

SUMMARY OF THE INVENTION An object of the present invention is to provide a wire rod for bearings and a method for manufacturing the same, which can shorten or omit the softening heat treatment required during cold working of parts for automobiles, construction, etc.

The wire rod for a bearing of the present invention has a weight percentage of 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: 0.01~0.06%, N: 0.02% or less (0 is excluded), the remainder consists of Fe and unavoidable impurities, and the prior austenite grain size of the microstructure is: It is characterized in that the sum of high-angle grain boundary lengths of 3 to 10 μm and 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 grain boundaries with a misorientation angle of 5° or less among the low-angle grain boundaries. is preferably 40 to 80%.
Preferably, the microstructure is composed of reticular pro-eutectoid cementite at the grain boundaries and pearlite within the grains.
The interlayer spacing within the pearlite can be between 0.05 and 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.
It is preferable that the average aspect ratio of cementite after one softening heat treatment is 2.5 or less.
The tensile strength after one-time softening heat treatment can be 750 MPa or less.

The method for producing a wire rod for a bearing according to the present invention includes, in weight percent, C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, Cr: A billet consisting of 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), and the rest is Fe and unavoidable impurities is heated at 950 to 1,050°C. heating in a temperature range of Ae1 to Acm°C, producing a wire rod by finishing hot rolling with a deformation amount equal to or greater than the critical deformation amount expressed by the following formula (1); It is characterized by including the step of cooling at a rate of 3° C./sec or more to a temperature range of 500 to 600° C., and then cooling at a rate of 1° C./sec or less.
Formula (1): -1.6Ceq ² +3.11Ceq -0.48
Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean the weight percent of each element.

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

The average size (AGS) of austenite grains before finish hot rolling is preferably 5 to 20 μm.
After cooling, the method may further include a softening heat treatment step of heating the wire at Ae1 to Ae1+40° C. and maintaining it for 5 to 8 hours.
After the softening heat treatment, the method may further include cooling to 660° C. at a rate of 20° C./hr or less.

According to the present invention, the wire rod for a bearing and the manufacturing method thereof according to the embodiments of the present invention can shorten or omit the softening heat treatment time, thereby reducing costs in the manufacturing process.

1 is a microstructure photograph taken using an optical microscope (OM) of a wire rod of Comparative Example 1 of the present invention before finishing hot rolling. 1 is a microstructure photograph taken with an optical microscope (OM) of the wire rod of Example 1 of the present invention before finish hot rolling. 1 is a microstructure photograph taken using a scanning electron microscope (SEM) of a wire rod of Comparative Example 1 of the present invention after finishing hot rolling and cooling. 1 is a microstructure photograph taken using a scanning electron microscope (SEM) of the wire rod of Example 1 of the present invention after finishing hot rolling and cooling. 1 is a photograph showing characteristics of grain boundaries observed through SEM-EBSD after finishing hot rolling and cooling a wire rod of Comparative Example 1 of the present invention. This is a photograph showing the characteristics of grain boundaries observed through SEM-EBSD after finishing hot rolling and cooling the wire of Example 1 of the present invention. 1 is a microstructure photograph taken using a scanning electron microscope (SEM) after subjecting the wire of Comparative Example 1 of the present invention to a spheroidizing heat treatment. 1 is a microstructure photograph taken using a scanning electron microscope (SEM) after subjecting the wire of Example 1 of the present invention to spheroidization heat treatment.

The wire rod for a bearing according to an embodiment of the present invention has a weight percentage of C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, and 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, and has a fine structure of prior austenite crystals. 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.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided so that the spirit of the invention will be fully conveyed to those skilled in the art to which the invention pertains. The invention is not limited to the embodiments presented here, but can be embodied in other forms. In the drawings, parts unrelated to the description are omitted to clarify the present invention, and the sizes of components are somewhat exaggerated to facilitate understanding.
Throughout the specification, when a part is described as "containing" a certain component, unless there is a statement to the contrary, this does not exclude other components, and may further include other components. means.
References to the singular include the plural unless the context clearly dictates otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
Wire rods for bearings may undergo spheroidization heat treatment to ensure workability. The spheroidization heat treatment is an additional process and requires a lot of heat treatment cost and time, which causes an increase in manufacturing costs.
The present inventors have conducted extensive research into ways to shorten or omit the spheroidizing and softening heat treatment in the production of bearing wire rods. As a result, the present invention was completed by confirming that the softening heat treatment time could be shortened or omitted by optimizing the alloy composition and manufacturing conditions and deriving the characteristics of grain boundaries.

The wire rod for a bearing according to an embodiment of the present invention has a weight percentage of C: 0.8 to 1.2%, Si: 0.01 to 0.6%, Mn: 0.1 to 0.6%, and Cr. : 1.0 to 2.0%, Al: 0.01 to 0.06%, N: 0.02% or less (0 is excluded), and the remainder consists of Fe and unavoidable impurities.
The role and content of each component contained in the bearing wire according to the present invention will be explained as follows. % 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%, it is difficult to ensure sufficient strength after the softening heat treatment and the quenching and tempering heat treatments performed after the forging process due to a decrease in the strength of the base material. On the other hand, if the content is too large, new precipitates such as M ₇ C ₃ may be formed and center segregation may occur during solidification of slabs such as blooms or billets, so the upper limit should be set at 1.2. %. Preferably, the content of C is 0.8-1.1%.

The content of Si is 0.01 to 0.6%.
Si (silicon) is a typical substitutional element, and is an element advantageous in ensuring strength through solid solution strengthening. If the Si content is less than 0.01%, it is difficult to ensure the strength and sufficient hardenability of the wire. On the other hand, if the content is too large, the strength may increase during forging after 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 forms a substitutional solid solution in the base structure to strengthen the base structure, and is an austenite-forming element that is added to ensure the target strength without reducing softness. If the Mn content is less than 0.1%, it is difficult to ensure the strength and toughness of the wire through solid solution strengthening. On the other hand, if the content of Mn, which is an austenite-forming element, is excessive, the cold Acm transformation point will be low during forging after softening heat treatment, and there is a risk that center segregation will occur and the wire structure will become non-uniform. The upper limit is limited to 0.6%.

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

The content of Al is 0.01-0.06%.
Aluminum (Al) not only has a deoxidizing effect, but also suppresses the growth of austenite grains by precipitating Al-based carbonitrides, and the content is 0.01% or more to ensure a pro-eutectoid ferrite fraction close to the equilibrium phase. Added. On the other hand, if the content is excessive, the generation of hard inclusions such as Al ₂ O ₃ will increase, and there is a risk that the inclusions will block the nozzle during continuous casting, so the upper limit should be set at 0.06%. limit.

The N content is 0.02% or less (0 is excluded).
Nitrogen (N) has a solid solution strengthening effect, but if its content is excessive, the toughness and softness of the material may deteriorate due to solid solution nitrogen that is not bonded with nitrides, so in the present invention, it is managed as an impurity. , its upper limit is limited to 0.02%.

The remaining component of the present invention is iron (Fe). However, in normal manufacturing processes, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be excluded. Examples of unavoidable impurities include P (phosphorus) and S (sulfur). These impurities are known to anyone skilled in the art of ordinary manufacturing processes, so all details thereof will not be specifically mentioned herein.
On the other hand, the microstructure of the bearing wire according to an embodiment of the present invention is composed of prior austenite crystal grains, with network-type pro-eutectoid cementite present at the grain boundaries and complete pearlite present within the grains.
Further, according to an embodiment of the present invention, the size of prior austenite grains in the microstructure is 3 to 10 μm.

During the softening heat treatment, the cementite in the pearlite structure changes its shape from a plate-like shape to a spherical shape, and the strength of the wire gradually decreases depending on the degree of spheroidization.
During the softening heat treatment, metal atoms move to various diffusion paths through the defect spaces in the material. ) and grain boundaries. Dislocations and grain boundaries have relatively large spaces compared to atomic defects, so they can diffuse at a high rate.
On the other hand, the heat treatment time during the softening heat treatment is determined by the diffusion rate of each atom, and the main factor that determines the diffusion rate is the grain boundary.

In the present invention, we attempt to separate high-angle grain boundaries and low-angle grain boundaries through misorientation between grains with grain boundaries in between, and control the distribution of each. did. Specifically, the mutual relationship with adjacent crystal grains was quantified by the misorientation angle value, and the grain boundaries were divided into high-angle grain boundaries of 15° or more and low-angle grain boundaries of 15° or less, based on 15°. The distribution of each crystal grain specified in the present invention is targeted not only in the surface layer of the wire but also in all regions up to the center.
In order to effectively shorten the softening heat treatment time, it is ideal to ensure a large amount of high-angle grain boundaries by making the grains as fine as possible and increasing the relative grain boundary area. In order to make the grains finer, problems arise in that the rolling load increases, equipment life is shortened, and productivity is reduced.

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

When the length distribution of low-angle grain boundaries is less than 250 mm/mm ² , the effect of reducing the softening heat treatment time is low, and when the length distribution of low-angle grain boundaries exceeds 800 mm/mm ² , As the dislocation density increases during rolling, recrystallization occurs partially and the dislocation density decreases, or the crystal grain size becomes non-uniform and bimodal with different sizes develops.
On the other hand, the smaller the misorientation angle, the more dislocations it contains; however, in the present invention, the proportion of grain boundaries with a misorientation angle of 5° or less among low-angle grain boundaries is 40 to 80%. be.

Next, a method for manufacturing a bearing wire, which is another aspect of the present invention, will be described in detail.
The wire rod of the present invention can be manufactured by manufacturing a billet having the above-mentioned alloy composition, and then subjecting the billet to a process of reheating, wire rod rolling, and multistage cooling.

Specifically, in the method for manufacturing a bearing wire according to another aspect of the present invention, 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 from Fe and unavoidable impurities. The step of heating the billet in the temperature range of 950 to 1,050°C, finish hot rolling in the 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) to produce a wire rod. and cooling the wire at a rate of 3° C./sec or more to a temperature range of 500 to 600° C., and then cooling the wire at a rate of 1° C./sec or less.
Formula (1): -1.6Ceq ² +3.11Ceq -0.48
Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean the weight percent of each element.
The reason for the numerical limitation of the alloying element content is as described above.

First, the present invention goes through the step of heating a billet having the above-mentioned composition components in a temperature range of 950 to 1,050°C.
If the heating temperature is less than 950° C., the load applied to the rolling rolls will increase, which may shorten the roll replacement cycle. On the other hand, if the heating temperature exceeds 1,050°C, rapid cooling is required for rolling, which not only makes it difficult to control the cooling, but also creates the risk of cracking, making it impossible to ensure good product quality. be.
Moreover, it is preferable that the heating is performed within 90 minutes. When heating is performed for more than 90 minutes, the depth of the decarburized layer on the surface of the wire becomes thick, and there is a possibility that the decarburized layer remains after rolling.

The heated billet is subjected to hot rolling in which rough rolling, intermediate rough rolling, finish rolling, and finish rolling are sequentially performed to produce a wire rod. The hot rolling is preferably groove rolling which makes the billet have the shape of a wire rod, and specifically, the billet is subjected to critical deformation expressed by the following formula (1) in a temperature range of Ae1 to Acm°C. The wire rod is manufactured by finishing hot rolling with a deformation amount greater than the amount of deformation.
During the production of wire rods, the rolling speed is so high that it falls under the dynamic recrystallization region. In the dynamic recrystallization regime, austenite grain size (AGS) depends only on deformation rate and deformation temperature. The present invention is characterized in that crystal grains are refined through dynamic recrystallization that occurs during rolling, and then the fine grains obtained during rolling are maintained as they are until room temperature through rapid cooling.

In order to refine grains during final finish rolling, the interpass time between the rolls is controlled to within 1 minute, and the austenite grain size (AGS) immediately before finish rolling is controlled within a range of 5 to 20 μm. It is preferable to maintain the finish rolling temperature at Ae1 to Acm° C. during finish rolling.
If the temperature during the finish hot rolling is Ae less than 1°C, the rolling load may increase and the equipment life may be shortened, while if it exceeds Acm°C, the temperature may not be maintained despite rapid cooling due to the high temperature. The maintenance time until the end of transformation becomes longer, and there is a possibility that the crystal grain refinement effect, which is the aim of the present invention, will be greatly reduced.

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

The present inventors took into account the correlation between Ceq and the amount of deformation and derived the critical amount of deformation expressed by equation (1).
The amount of deformation is defined as -ln(1-RA), where RA is the reduction rate due to the rolling pass (RA<1). If the deformation amount has not reached the critical deformation amount, the reduction amount is not sufficient and it is difficult to sufficiently refine the crystal grains in the center of the wire, which has a negative effect on the spheroidization behavior of the wire during softening heat treatment. effect.

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

If the Tpf-Tf value exceeds 50°C, the deviation of the wire microstructure becomes so large that a uniform microstructure cannot be ensured, and the wire surface is overcooled and a hard phase is generated or crystal grains are formed. There is a risk that the area may become coarse.
After hot rolling in the above temperature range, cooling at a rate of 3°C/sec or more to a temperature range of 500 to 600°C, and then cooling at a rate of 1°C/sec or less to produce the bearing wire of the present invention. can be manufactured.

The above cooling step is an essential step to ensure fine grain distribution, and in the present invention, the heat treatment time can be shortened through accelerated diffusion by controlling the cooling end temperature and cooling rate. This is 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 grains obtained through hot rolling below the transformation point, and the misorientation angle is 15° or less. There is a possibility that the fraction of low-angle grain boundaries will decrease significantly. On the other hand, if the cooling rate after reaching the 500-600°C temperature range exceeds 1°C/sec, low-temperature structures such as bainite will occur and softening will not proceed sufficiently despite the spheroidizing heat treatment. There is a possibility.
Next, the method may further include a step of winding the wire rod after the cooling step and then subjecting the wire rod to a softening heat treatment.

In the softening heat treatment process, various heat treatment patterns can be applied depending on the degree of softening required at a temperature around Ae 1° C. of the wire. In the present invention, after cooling, the wire was subjected to a softening heat treatment in which the wire was heated to Ae1 to Ae1+40°C and maintained for 5 to 8 hours.
When the heating temperature is Ae less than 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 number of spheroidized carbide seeds decreases, making it impossible to obtain a sufficient softening heat treatment effect. Further, the heating is preferably performed for 5 to 8 hours. When heating for more than 8 hours, there is a problem that the cost of the manufacturing process increases. On the other hand, when heating for less than 5 hours, the heat treatment does not proceed sufficiently, resulting in a problem that the aspect ratio of cementite increases.

After the softening heat treatment step, a cooling step is performed to 660° C. at a rate of 20° C./hr or less. 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, it is possible to secure 80% or more of carbide having an average cementite aspect ratio of 2.5 or less not only in the surface layer of the wire but also in the entire area up to the center.

In the present invention, the tensile strength of the wire can be controlled to be as low as 740 MPa or less with just one softening heat treatment, so cold heading or cold forging for manufacturing the final product is easy. This makes it possible to shorten or omit the time for spheroidization heat treatment, which is an additional step after wire rod manufacturing, and thus enables cost savings.
Hereinafter, the present invention will be explained 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 to the present invention is determined by the matters stated in the claims and matters reasonably inferred therefrom.
Example

A billet was produced by casting a steel material having the composition shown in Table 1 below, 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 (hereinafter referred to as "AGS") before finish rolling was measured through a cut plane (crop) performed before finish hot rolling. Moreover, Tpf is the average surface temperature of the wire rod before finish rolling, and Tf is the average surface temperature of the wire rod after finish rolling.

Thereafter, the microstructure, grain boundary characteristics, and mechanical properties (tensile strength, area reduction rate) of each of the manufactured examples and comparative examples were measured and 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 method, and then performing a tensile test.
RA stands for Reduction Ratio, which is a measure of the change in cross-sectional area of a tensile test piece that was broken during a tensile test of the material, and is a numerical expression of the softness of the material.

Average grain size (AGS) was measured using the ASTM E112 method. After producing a wire rod by hot rolling, the non-water-cooled part was removed and the specimen was taken, and measurements were taken at three arbitrary points on the surface, at a point 1/4 from the diameter, and at a point 1/2 from the diameter. Shown as average value.
The characteristics of grain boundaries can be determined by taking a specimen using the same method as the grain size (AGS) measurement method, and using SEM-EBSD to measure the surface, 1/4 point from the diameter, and 1/2 point from the diameter at x700. An area of 130×130 μm ² was measured at a magnification of 0.1 μm step-size, and the average value is shown. The average value of Confidence Index was 0.57 or more.

On the other hand, the wire rods of each example and comparative example were subjected to a spheroidization 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 spheroidization heat treatment was performed on the manufactured wire test piece without the steps of primary softening treatment and primary wire drawing, and the presence or absence of spheroidization was determined.
At this time, the average aspect ratio of the cementite in the wire after the spheroidization heat treatment is determined by photographing three fields of view with a 3000x SEM in the 1/4 to 1/2 area in the diameter direction of the wire, and using an image measurement program to determine the cementite within the field of view. The long axis/shortening is automatically measured and then statistically processed and displayed.
The presence or absence of spheroidization is determined by randomly photographing 10 or more locations using an SEM electron microscope and then observing the carbides in a 5,000x field of view. Spheroidization was determined to have occurred when the occupancy rate of carbonized carbide was 80% or more.

Comparative Examples 1 to 4 are examples in which the alloy composition satisfies the specifications specified in the present invention, but the positive manufacturing process conditions described below deviate from the present invention, and are described as comparative examples.

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

As shown in Table 3, the wire rods of Examples 1 to 3 that satisfy the alloy composition and manufacturing conditions proposed by the present invention have prior austenite grain sizes (AGS) of 3 to 10 μm, and misorientation angles of 3 to 10 μm. ) was 15° or more, and the length distribution of high-angle grain boundaries was 1,000 to 4,000 mm/mm ² , making it possible to secure fine crystal grains. Further, the wire rods of Examples 1 to 3 ensured a high tensile strength of 1,200 MPa or more compared to the comparative example, and the cross-sectional area reduction rate was 20% or more.

5 and 6 are photographs of the characteristics of grain boundaries observed through SEM-EBSD after finishing hot rolling and cooling the wire rods of Example 1 of the present invention and Comparative Example 1, respectively.
As shown in FIGS. 5 and 6, Example 1 has a higher degree of distribution of low-angle grain boundaries where the misorientation angle shown in green and red is 15° or less compared to Comparative Example 1. It can be confirmed that the
As shown in Table 4, the wire rods of Examples 1 to 3 that 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 one softening heat treatment, but also have a low tensile strength of 740 MPa or less. By securing fine crystal grains, it was possible to secure spheroidized cementite with an average aspect ratio of 2.5 or less even with a shorter spheroidization heat treatment than the conventional heat treatment of 30 hours or more.

FIG. 7 and FIG. 8 are microstructure photographs taken with a scanning electron microscope (SEM) after subjecting the wire rods of Example 1 of the present invention and Comparative Example 1 to spheroidization heat treatment, respectively.
As shown in FIGS. 7 and 8, in Example 1, the spheroidal cementite was more uniformly distributed than in Comparative Example 1, so it can be confirmed that spheroidization was performed at a faster rate.
In the case of Comparative Example 1, the Mn content was excessive, and as the Acm transformation point rose, the crystal grains were not sufficiently refined during rolling. As a result, even after the softening heat treatment, the cementite average aspect ratio was 8.5, making it impossible to obtain a spheroidized structure, 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 higher than the Acm°C transformation point, and the cooling time required to complete the phase transformation became longer, resulting in a significant reduction in the grain refinement effect. . 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 component range specified by the present invention is satisfied, but the Tpf-Tf value greatly exceeds 85°C and 50°C, and the temperature deviation between the inside and outside of the material increases 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, making it impossible to obtain a spheroidized structure, and the tensile strength value was as high as 810 MPa.

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

Although the exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary knowledge in the relevant technical field will understand the concept and scope of the following claims. It should be understood that various changes and modifications can be made within the scope of the invention.

The bearing wire rod and the manufacturing method thereof according to the present invention can shorten or omit the softening heat treatment time, thereby reducing costs in the manufacturing process.

Claims (12)

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,
The prior austenite grain size of the microstructure is 3 to 10 μm,
A wire rod for a bearing, characterized in that 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. 2. The wire rod for a bearing according to claim 1, wherein the microstructure is composed of reticular pro-eutectoid cementite at the grain boundaries and pearlite within the grains. The bearing wire according to claim 2 , wherein the interlayer spacing within the pearlite is 0.05 to 0.2 μm. The bearing wire 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. Regarding the softening heat treatment performed after the cooling step, in which the wire is heated to Ae1 to Ae1 + 40°C and maintained for 5 to 8 hours, the average aspect ratio of the cementite after the first softening heat treatment is 2.5 or less. The bearing wire according to claim 1. Regarding the softening heat treatment performed after the cooling step, in which the wire is heated to Ae1 to Ae1+40°C and maintained for 5 to 8 hours, the tensile strength after one softening heat treatment is 750 MPa or less. Wire rod for bearings listed. A method for manufacturing a wire rod for bearings, the method comprising:
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: heating a billet consisting of 0.01 to 0.06%, N: 0.02% or less (0 is excluded), the remainder being Fe and unavoidable impurities in a temperature range of 950 to 1,050 ° C.;
A step of producing a wire rod by finish hot rolling at a temperature range of Ae1 to Acm°C with a deformation amount equal to or more than the critical deformation amount expressed by the following formula (1), and rolling the wire rod at a speed of 3°C/sec or more. After cooling to a temperature range of 500 to 600°C, cooling at a rate of 1°C/sec or less ,
The bearing wire rod is
The prior austenite grain size of the microstructure is 3 to 10 μm, and the sum of high-angle grain boundary lengths with a misorientation angle of 15° or more is 1,000 to 4,000 mm/mm per unit area. ^2. A method for producing a bearing wire material, characterized in that :
Formula (1): -1.6Ceq ² +3.11Ceq -0.48
(Here, Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr mean the weight percent of each element.) 8. The method for manufacturing a bearing wire rod according to claim 7 , wherein the wire rod satisfies the following formula (2).
Formula (2): Tpf-Tf≦50°C
(Here, Tpf is the average surface temperature of the wire rod before finish hot rolling, and Tf is the average surface temperature of the wire rod after finish hot rolling.) 8. The method for manufacturing a bearing wire according to claim 7 , wherein the heating time is 90 minutes or less. The method for producing a bearing wire rod according to claim 7 , wherein the average austenite grain size (AGS) before finish hot rolling is 5 to 20 μm. The method of manufacturing a bearing wire according to claim 7 , further comprising a softening heat treatment step of heating the wire at Ae1 to Ae1+40° C. for 5 to 8 hours after cooling. The method for manufacturing a bearing wire according to claim 11 , further comprising the step of cooling to 660°C at a rate of 20°C/hr or less after the softening heat treatment.

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