KR20220101598A - Wire rod for bearing and methods for manufacturing thereof - Google Patents

Abstract

The present invention provides a wire rod for bearings capable of reducing the time for spheroidization and a manufacturing method thereof. The wire rod for bearings according to an embodiment of the present invention comprises 0.8 to 1.2 wt% of C, 0.01 to 0.6 wt% of Si, 0.1 to 0.6 wt% of Mn, 1.0 to 2.0 wt% of Cr, 0.01 to 0.06 wt% of Al, 0.02 wt% or less (excluding 0 wt%) of N, and the balance Fe and unavoidable impurities, the grain size of prior austenite in the microstructure is 3 to 10 μm, and the sum of the lengths of high angle grain boundaries, where the misorientation angle is 15° or greater, per unit area is 1,000 to 4,000 mm/mm^2.

Description

Wire rod for bearing and manufacturing method thereof

The present invention relates to a bearing wire rod and a method for manufacturing the same, and more particularly, to a bearing wire rod and a method for manufacturing the same applicable to automobiles, construction parts, etc. by shortening and omitting subsequent softening heat treatment.

As the carbon content of the wire rod increases, the strength of the material rapidly increases, making direct forming and processing difficult, and the ductility or toughness of the material rapidly decreases due to proeutectoid cementite precipitated along the prior austenite grain boundary during cooling.

In order to soften the wire rod, generally spheroidizing heat treatment is performed. Spheroidizing heat treatment spheroidizes cementite and induces homogeneous particle distribution in order to improve cold workability during cold forming. In addition, in order to improve the lifetime of the machining die, the hardness of the material to be machined may be lowered.

On the other hand, cold-rolling wire (CHQ) adopts wire-drawing first to accelerate spheroidization, but wire for bearings with relatively high carbon content has a problem in that when wire-drawing is first introduced, disconnection due to internal defects occurs. .

In general, in order to manufacture a wire rod for bearing steel into a steel wire, it is subjected to one or more softening heat treatments. Afterwards, in order to improve cold forgeability, wire drawing and heat treatment processes are additionally performed, and cold forgeability is secured by tensile strength and spheroidization rate after soft nitriding heat treatment.

However, in order to soften the bearing wire rod, it takes a long time of 30 hours or more at a high temperature of 700 to 800° C. Accordingly, development of a bearing wire rod capable of shortening or omitting an additional softening heat treatment process and a method for manufacturing the same is required.

An object of the present invention is to provide a wire rod for a bearing capable of shortening or omitting soft nitriding heat treatment required for cold working of automobiles and construction parts, and a method for manufacturing the same.

The wire rod for bearing according to an embodiment of the present invention is, 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 (excluding 0), containing the remaining Fe and unavoidable impurities, the prior austenite grain size of the microstructure is 3 to 10 μm, and the high-hardness grain boundary having a misorientation angle of 15° or more The sum of the lengths is 1,000 to 4,000 mm/mm ² per unit area.

In addition, according to an embodiment of the present invention, the sum of the lengths of the low-inclination grain boundaries having an azimuth angle of less than 15° is 250 to 800 mm/mm ² per unit area, and the ratio of the grain boundaries having an azimuth angle of 5° or less among the low-inclination grain boundaries. may be 40 to 80%.

In addition, according to an embodiment of the present invention, the microstructure may be composed of reticulated proeutectoid cementite at the grain boundary and pearlite within the grain.

In addition, according to an embodiment of the present invention, the layer spacing in the pearlite may be 0.05 to 0.2㎛.

Further, according to an embodiment of the present invention, the tensile strength may be 1,200 MPa or more, and the cross-sectional area reduction ratio (RA) may be 20% or more.

In addition, according to an embodiment of the present invention, after one-time softening heat treatment, the average aspect ratio of cementite may be 2.5 or less.

In addition, according to an embodiment of the present invention, after one-time softening heat treatment, the tensile strength may be 750 MPa or less.

According to another embodiment of the present invention, the method of manufacturing a bearing wire rod according to 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 (excluding 0), heating the billet containing the remaining Fe and unavoidable impurities in a temperature range of 950 to 1,050 ℃; manufacturing a wire rod by finish hot rolling in a temperature range of Ae1 to Acm°C with a deformation amount greater than or equal to the critical deformation amount expressed by the following formula (1); and cooling the wire rod to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and then cooling the wire rod at a rate of 1° C./sec or less.

Equation (1): -1.6Ceq ² + 3.11Ceq - 0.48

Here, Ceq = C + Mn/6 + Cr/5, and C, Mn, and Cr mean weight % of each element.

In addition, according to an embodiment of the present invention, the wire rod may satisfy the following equation (2).

Equation (2): Tpf - Tf ≤ 50°C

Here, Tpf is the average surface temperature of the wire rod before finishing hot rolling, and Tf is the average surface temperature of the wire rod after finishing hot rolling.

In addition, according to an embodiment of the present invention, the heating time may be 90 minutes or less.

In addition, according to an embodiment of the present invention, the austenite grain average size (AGS) before the finish hot rolling may be 5 to 20㎛.

In addition, according to an embodiment of the present invention, after cooling, a softening heat treatment step of heating the wire rod to Ae1 to Ae1+40° C. and maintaining it for 5 to 8 hours; may further include.

In addition, according to an embodiment of the present invention, after the softening heat treatment, cooling to 660°C at a rate of 20°C/hr or less; may further include.

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

1 and 2 are microstructure photographs taken with an optical microscope (OM) before finishing hot rolling of the wire rods of Example 1 and Comparative Example 1 of the present invention, respectively.
3 and 4 are microstructure photographs taken with a scanning electron microscope (SEM) after finishing hot rolling and cooling of the wire rods of Example 1 and Comparative Example 1 of the present invention, respectively.
5 and 6 are photographs of observing grain boundary characteristics 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.
7 and 8 are microstructure photographs taken with a scanning electron microscope (SEM) after spheroidizing the wire rods of Example 1 and Comparative Example 1 of the present invention, respectively.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented in order to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein, and may be embodied in other forms. The drawings may omit the illustration of parts irrelevant to the description in order to clarify the present invention, and may slightly exaggerate the size of the components to help understanding.

Throughout the specification, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

The singular expression includes the plural expression unless the context clearly dictates otherwise.

Hereinafter, an embodiment according to the present invention will be described in detail with reference to the accompanying drawings.

Bearing wire rods are sometimes subjected to spheroidizing heat treatment to secure workability. The spheroidizing heat treatment is an additional process, and since it takes a lot of heat treatment cost and time, it causes an increase in manufacturing cost.

The present inventors have studied in depth a method for shortening or omitting the spheroidization softening heat treatment in manufacturing the wire rod for a bearing. As a result, it was confirmed that the softening heat treatment time can be shortened or omitted by deriving the characteristics of grain boundaries by optimizing the alloy composition and manufacturing conditions, and the present invention was completed.

The wire rod for bearing according to an aspect of the present invention is, 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 (excluding 0), including remaining Fe and unavoidable impurities.

Hereinafter, the role and content of each component included in the wire rod for bearing according to the present invention will be described as follows. % for the following components means % by weight.

The content of C is 0.8 to 1.2%.

C (carbon) is an element added to secure product strength. When the content of C is less than 0.8%, it is difficult to secure sufficient strength after the quenching and tempering heat treatment that is performed after the softening heat treatment and forging process due to the decrease in strength of the base material. However, when the content is excessive, new precipitates such as M ₇ C ₃ are formed, and there is a problem that central segregation occurs during solidification of cast slabs such as blooms or billets, so the upper limit may be limited to 1.2%. Preferably, the content of C is 0.8 to 1.1%.

The content of Si is 0.01 to 0.6%.

Si (silicon) is a representative substitution-type element and is advantageous in securing strength through solid solution strengthening. When the Si content is less than 0.01%, it is difficult to secure the strength and sufficient hardenability of the wire rod. However, when the content is excessive, there is a problem in that it is difficult to secure cold forgeability due to an increase in strength during forging after soft nitriding heat treatment, and the upper limit thereof may be limited to 0.6%.

The content of Mn is 0.1 to 0.6%.

Mn (manganese). It is an element that strengthens solid solution by forming a substitutional solid solution in the matrix structure. It is an austenite forming element added to secure the desired strength without reducing ductility. When the Mn content is less than 0.1%, it is difficult to secure strength and toughness by solid solution strengthening of the wire rod. However, if the content of Mn, which is an austenite forming element, is excessive, the cold Acm transformation point is lowered during forging after soft nitriding heat treatment, and there is a problem that the structure of the wire rod becomes non-uniform due to center segregation. can

The content of Cr is 1.0 to 2.0%.

Cr (chromium), like Mn, is an element advantageous for securing a martensitic structure by improving the hardenability of the wire rod. When the content of Cr is less than 1.0%, it is difficult to obtain a martensitic microstructure during quenching and tempering heat treatment performed after soft nitriding heat treatment and forging process. However, when the content is excessive, there is a problem in that a large amount of low-temperature structure is formed in the wire rod due to central segregation, and the upper limit thereof may be limited to 2.0%.

The content of Al is 0.01 to 0.06%.

Aluminum (Al) is added in an amount of 0.01% or more to suppress the growth of austenite grains by precipitating Al-based carbonitride as well as deoxidation effect, and to secure the proeutectoid ferrite fraction close to the equilibrium phase. However, when the content is excessive, the occurrence of hard inclusions such as Al ₂ O ₃ increases, and there is a problem in that nozzles are clogged by inclusions, particularly during playing, and the upper limit can be limited to 0.06%.

The content of N is 0.02% or less (excluding 0).

Nitrogen (N) has a solid solution strengthening effect, but if its content is excessive, there is a problem in that the toughness and ductility of the material are inferior due to the solid solution nitrogen not combined with nitride, and it is managed as an impurity in the present invention, and the upper limit is 0.02% can be limited to

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. As an unavoidable impurity, P (phosphorus), S (sulfur), etc. are mentioned, for example. Since these impurities are known to anyone skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

On the other hand, the microstructure of the wire rod for bearing according to an embodiment of the present invention is along the old austenite grains, the reticulated proeutectoid cementite is present at the grain boundary, and complete pearlite is present in the grains.

In addition, according to an embodiment of the present invention, the prior austenite grain size of the microstructure is 3 to 10 μm.

During the softening heat treatment, the cementite in the pearlite structure changes from plate to spherical shape, and the strength of the wire rod gradually decreases according to the degree of spheroidization.

During soft nitriding heat treatment, metal atoms move through various diffusion paths through the defect space in the material. spread through Because the space of dislocations and grain boundaries is relatively wide compared to atomic defects, rapid diffusion is possible.

On the other hand, in the case of soft nitriding heat treatment, the heat treatment time is determined by the diffusion rate of each atom, and the most important factor controlling the diffusion rate is the grain boundary.

In the present invention, a high-hardness grain boundary and a low-hardness grain boundary are distinguished through a misorientation between grain boundaries in a grain boundary structure, and each distribution is controlled. Specifically, the interrelationship with neighboring grains was quantified as a misorientation angle value, and 15° was divided into high-hardness grain boundaries greater than 15° and low-inclination grain boundaries less than 15°. The distribution of each crystal grain specified in the present invention corresponds not only to the surface layer portion of the wire rod, but also to the entire region up to the center portion.

In order to effectively shorten the softening heat treatment time, it is ideal to secure a large amount of high-hardness grain boundaries by increasing the relative grain boundary area by refining the crystal grains as much as possible. There is a problem of degradation.

Accordingly, in the present invention, the total length per unit area of the high-hardness grain boundary having an orientation difference angle of 15° or more while controlling the prior austenite grain size was attempted. Specifically, the prior austenite grain size (AGS) of the wire rod for bearing according to the disclosed embodiment is 3 to 10 μm, and the sum of the high hardness grain boundary lengths having an azimuth angle of 15° or more is 1,000 to 4,000 mm/mm per unit area ² to be.

On the other hand, the low-hardness grain boundary with an azimuth angle of less than 15° distributed within the high-hardness grain boundary is a place where the dislocations generated by deformation during hot rolling gather, and it can contribute to the improvement of cold forgeability by helping the spheroidizing behavior during soft nitriding heat treatment. have. In the present invention, the sum of the lengths of the low angle grain boundaries having an azimuth angle of less than 15° is 250 to 800 mm/mm ² per unit area.

When the length distribution of the low-angle grain boundary is less than 250mm/mm ² , the effect of shortening the softening heat treatment time is insignificant, and when the length distribution of the low-angle grain boundary exceeds 800mm/mm ² , the dislocation density during rolling increases There is a problem in that the dislocation density is rather decreased due to partial recrystallization, or the grain size is not uniform and develops into a bimodal form of different sizes.

On the other hand, the smaller the azimuth angle means, the more dislocations are included. In the present invention, the ratio of grain boundaries having an azimuth angle of 5° or less among low-inclination grain boundaries is 40 to 80%.

Next, a method for manufacturing a wire rod for a bearing, which is another aspect of the present invention, will be described in detail.

The wire rod of the present invention may be manufactured by manufacturing a billet having the above-described alloy composition, and then reheating it - rolling the wire rod - and cooling it in multiple stages.

Specifically, the method of manufacturing a wire rod for a spring according to another aspect of the present invention is, 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 (excluding 0), heating the billet containing the remaining Fe and unavoidable impurities in a temperature range of 950 to 1,050 ℃; manufacturing a wire rod by finish hot rolling in a temperature range of Ae1 to Acm°C with a deformation amount greater than or equal to the critical deformation amount expressed by the following formula (1); and cooling the wire rod to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and then cooling the wire rod at a rate of 1° C./sec or less.

Equation (1): -1.6Ceq ² + 3.11Ceq - 0.48

Here, Ceq = C + Mn/6 + Cr/5, and C, Mn, and Cr mean weight % of each element.

The explanation for the reason for limiting the numerical value of the alloying element content is as described above.

First, in the present invention, a step of heating the billet having the above-described compositional components in a temperature range of 950 to 1,050° C. is performed.

When the heating temperature is less than 950° C., the load applied to the rolling roll becomes large, and thus there is a problem in that the roll replacement cycle is shortened. On the other hand, when the heating temperature exceeds 1,050° C., rapid cooling is required for rolling, so it is difficult to control cooling, and cracks occur, so that good product quality cannot be secured.

In addition, the heating is preferably performed for 90 minutes or less. 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 problem in that the decarburized layer remains after the end of rolling.

A wire rod is manufactured by performing hot rolling sequentially consisting of rough rolling, intermediate rough rolling/finishing rolling, and finish rolling on a heated billet. The hot rolling is preferably a ball rolling in which the billet has the shape of a wire rod. Specifically, the billet is finished hot-rolled in a temperature range of Ae1 to Acm°C, with a deformation amount greater than or equal to the critical deformation amount expressed by the following formula (1) to finish hot rolling the wire rod. to manufacture

When manufacturing a wire rod, the rolling speed is very fast, which corresponds to a dynamic recrystallization region. In the dynamic recrystallization region, the austenite grain size (AGS) depends only on the strain rate and strain temperature. In the present invention, it was attempted to refine crystal grains through dynamic recrystallization occurring during rolling, and then to maintain fine grains secured during rolling through rapid cooling to room temperature.

In order to refine the crystal grains during the final finishing rolling, the interpass time between the rolls is controlled within 1 minute to secure the austenite grain size (AGS) immediately before the finish rolling in the range of 5 to 20 μm, and then finishing It is preferable to control the finish rolling temperature to Ae1 to Acm℃ during rolling.

If the temperature during the finish hot rolling is less than Ae1 ℃, there is a problem that the rolling load increases and the equipment life is shortened. On the other hand, if it exceeds Acm ℃, the holding time until the end of the phase transformation is prolonged even after rapid cooling due to the high temperature, in the present invention There is a problem in that the desired crystal grain refining effect is greatly reduced.

In addition, the deformation amount during hot rolling in the above temperature range can be controlled to be greater than or equal to the critical deformation amount expressed by the following formula (1).

Equation (1): -1.6Ceq ² + 3.11Ceq - 0.48

Here, Ceq = C + Mn/6 + Cr/5, and C, Mn, and Cr mean weight % of each element.

The present inventors derived the critical deformation amount expressed by Equation (1) in consideration of the correlation between Ceq and the amount of deformation.

The amount of deformation is defined as -ln(1-RA), where RA is the reduction in area by the rolling pass (RA<1). When the amount of deformation is less than the critical deformation amount, it is difficult to sufficiently refine the crystal grains in the center of the wire rod because the reduction amount is not sufficient, which adversely affects the spheroidization behavior of the wire rod during soft nitriding heat treatment.

On the other hand, the wire rod during hot rolling satisfies the following formula (2).

Equation (2): Tpf - Tf ≤ 50°C

Here, Tpf is the average surface temperature of the wire rod before finishing hot rolling, and Tf is the average surface temperature of the wire rod after finishing hot rolling.

When the Tpf - Tf value exceeds 50° C., the deviation of the wire rod microstructure becomes very large, so that a uniform microstructure cannot be secured, and there is a problem in that a hard phase occurs due to supercooling on the surface of the wire rod or the grains are coarsened.

After hot rolling in the above-described temperature range, 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 to manufacture the bearing wire of the present invention. can

The above-described cooling step is an essential process in order to secure a fine grain distribution, and in the present invention, the cooling end temperature and cooling rate are controlled to secure a microstructure capable of shortening the heat treatment time through diffusion acceleration.

When the cooling rate up to the temperature range of 500 to 600°C is less than 3°C/sec, it is difficult to maintain the fine grains secured through hot rolling to below the transformation point, and the fraction of low-hardness grain boundaries with an azimuth angle of less than 15° is greatly reduced there is a problem with On the other hand, when the cooling rate after reaching the temperature range of 500 to 600 ° C is more than 1 ° C / sec, low-temperature structures such as bainite are generated, and there is a problem that soft nitriding does not proceed sufficiently despite the spheroidizing heat treatment.

Next, after winding the wire rod that has undergone the cooling step, a softening heat treatment step; may be further included.

In the softening heat treatment process, various heat treatment patterns can be applied according to the degree of softening required at a temperature near Ae1°C of the wire rod. In the present invention, after cooling, the wire rod is heated to Ae1 to Ae1+40° C. and soft nitriding heat treatment is performed for 5 to 8 hours.

When the heating temperature is less than Ae1 ℃, there is a problem in that the softening heat treatment time becomes long. On the other hand, when the heating temperature exceeds Ae1+40°C, the number of spheroidized carbide seeds is reduced and a sufficient softening heat treatment effect cannot be obtained. In addition, the heating is preferably performed for 5 to 8 hours. In the case of heating for more than 8 hours, there is a problem in that the manufacturing process cost increases. On the other hand, when heating for less than 5 hours, there is a problem that the aspect ratio of cementite increases because the heat treatment does not proceed sufficiently.

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, when the cooling rate exceeds 20° C./hr, there is a problem in that pearlite is formed again due to the excessive cooling rate.

After performing the softening heat treatment, the tensile strength of the wire rod may be 750 MPa or less, and the average aspect ratio of cementite in the wire rod may be 2.5 or less. Specifically, it is possible to secure 80% or more of carbides having an average aspect ratio of cementite of 2.5 or less in the entire area up to the center as well as the surface layer of the wire rod.

In the present invention, since the tensile strength of the wire rod can be controlled as low as 740 MPa or less with only one soft nitriding heat treatment, cold forging or cold forging for manufacturing a final product is easy. Accordingly, it is possible to shorten or omit the spheroidizing heat treatment time, which is an additional process after manufacturing the wire, thereby reducing costs.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Example

A billet was manufactured 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 to prepare 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 cutting crop performed before finish hot rolling. In addition, T _pf is the average surface temperature of the wire rod before finishing rolling, and T _f is the average surface temperature of the wire rod after finishing rolling.

steel grade alloy composition Formula (1) C Si Mn Cr Al N -1.6Ceq ² + 3.11Ceq - 0.48 Invention lecture 1 0.98 0.32 0.45 1.45 0.035 0.015 0.81 Invention lecture 2 1.05 0.24 0.51 1.50 0.023 0.001 0.69 Invention lecture 3 0.98 0.25 0.45 1.43 0.035 0.015 0.81 Comparative lecture 1 1.20 0.25 0.75 2.00 0.005 0.005 0.12 Comparative lecture 2 0.93 0.25 0.33 1.22 0.005 0.005 0.93

steel grade Heating temperature (℃) /
Heating time (min) Average AGS before finish rolling (㎛) Finish rolling temperature (℃) deformation amount T _pf - T _f (°C) Cooling rate up to 500℃
(℃/s) Cooling rate after 500℃
(℃/s) Example 1 Invention lecture 1 950/90 7 760 1.2 40 5 0.5 Example 2 Invention lecture 2 1,000/80 11 750 0.8 38 4 One Example 3 Invention lecture 3 1,020/90 9 730 0.95 43 6 0.7 Comparative Example 1 Comparative lecture 1 1,000/90 15 780 0.1 44 2 3 Comparative Example 2 Comparative lecture 2 950/80 11 850 0.6 63 4 2 Comparative Example 3 Invention lecture 1 1,100/90 24 880 0.85 85 One One Comparative Example 4 Invention lecture 2 1,000/90 13 770 0.32 55 3 2

Thereafter, the microstructure and grain boundary characteristics and mechanical properties (tensile strength, reduction in area) of each of the prepared Examples and Comparative Examples were measured and shown in Table 3 below. Tensile strength was measured by processing a tensile specimen in accordance with the ASTM E8 standard on a hot-rolled wire rod, followed by a tensile test according to the method for manufacturing a steel wire described above.

RA means the reduction ratio, and it is a measure of the change in cross-sectional area in a tensile specimen that is fractured during a tensile test of a material and expresses the ductility of the material as a numerical value.

The average grain size (AGS) was measured using the ASTM E112 method. After the wire rod was prepared by hot rolling, the uncooled part was removed, and three arbitrary points were measured on the surface, 1/4 point from the diameter, and 1/2 point from the diameter, respectively, and expressed as an average value.

The grain boundary characteristics are 130 x 130 at a magnification of x700 at the surface, 1/4 point from the diameter, and 1/2 point from the diameter using SEM-EBSD after taking a specimen in the same way as the grain size (AGS) measurement method. The area of μm ² was measured with a 0.1 μm step-size and expressed as an average value, and the average value of the Confidence Index was 0.57 or more.

Microstructure and grain boundary characteristics mechanical properties AGS
(μm) lamellar spacing
(μm) ≥15°
grain boundary length distribution
(mm/mm ² ) <15°
grain boundary length distribution
(mm/mm ² ) <15° of grain boundaries ≤5° of grain boundaries (%) Tensile strength (MPa) Cross-sectional area reduction rate
(%) Example 1 4 0.12 2500 420 60 1250 25 Example 2 5.5 0.11 3500 650 55 1260 32 Example 3 5 0.15 3700 550 63 1210 27 Comparative Example 1 12 0.21 2150 210 35 1020 13 Comparative Example 2 11 0.22 850 120 17 980 11 Comparative Example 3 15 0.29 1450 150 22 1020 14 Comparative Example 4 13 0.21 1200 160 25 1030 13

Meanwhile, after spheroidizing the wire rods of each Example and Comparative Example once under the conditions of Table 4 below, 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 without the primary softening treatment and the primary wire drawing process on the specimen of the prepared wire rod, and whether spheroidization was determined. At this time, after spheroidizing heat treatment, the average aspect ratio of cementite of the wire rod is taken in a 3 field of view of 1/4 to 1/2 in the radial direction of the wire, and the long/short axis of cementite in the field of view is automatically measured using an image measurement program. It is measured through statistical processing after measurement.

Judgment of whether or not spheroidization is randomly selected through an SEM electron microscope at 10 or more, and among all carbides observed at a × 5,000 field of view, if the occupancy of spheroidized carbides with an aspect ratio of 2.5 or less is 80% or more, spheroidization is made. judged.

division Ae1(℃) Heat treatment temperature (℃) Heat treatment time (Hr) Cooling rate up to 660℃ (℃/Hr) Cementite average aspect ratio after heat treatment Tensile strength after heat treatment (MPa) Example 1 743.6 765 8 15 1.6 720 Example 2 741.6 780 7 17 2.1 733 Example 3 739.7 770 6 10 1.5 730 Comparative Example 1 738.4 700 7 30 8.5 820 Comparative Example 2 734.8 740 10 20 6.2 790 Comparative Example 3 740.2 800 7.5 15 7.5 810 Comparative Example 4 740.2 765 4 25 5.5 770

Comparative Examples 1 to 4, although the alloy composition satisfies the suggestion of the present invention, the following manufacturing process conditions are out of the present invention, so it is indicated as a comparative example. 1 and 2 are microstructure photographs taken with an optical microscope (Optical Microscope, OM), respectively, of the wire rods of Example 1 and Comparative Example 1 of the present invention before finishing hot rolling, and FIGS. 3 and 4 are, respectively, of the present invention. Example 1, Comparative Example 1 After finishing hot rolling and cooling the wire rod, it is a microstructure photograph taken with a scanning electron microscope (SEM).

1 to 4, Example 1 has a relatively fine prior austenite grain size (AGS) before finish hot rolling compared to Comparative Example 1, and thus it can be confirmed that the grains are fine even after finish hot rolling and cooling. have.

Referring to Table 3, the wire rods of Examples 1 to 3 satisfying the alloy composition and manufacturing conditions proposed by the present invention had a prior austenite grain size (AGS) of 3 to 10 μm, and a misorientation angle. The length distribution of the high-hardness grain boundary of 15° or more was 1,000 to 4,000 mm/mm ² , and it was possible to secure fine grains. In addition, the wire rods of Examples 1 to 3 showed a reduction in cross-sectional area of 20% or more while securing a high tensile strength of 1,200 MPa or more compared to Comparative Examples.

5 and 6 are photographs of observing grain boundary characteristics 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.

Referring to FIGS. 5 and 6 , it can be seen that Example 1 has a higher distribution of low-inclination-angle grain diameters with a misorientation angle of less than 15°, indicated in green and red, compared to Comparative Example 1.

Referring to Table 4, the wire rods of Examples 1 to 3 satisfying the alloy composition and manufacturing conditions proposed by the present invention not only have a low tensile strength of 740 MPa or less, but also secure fine grains after one-time softening heat treatment. By doing so, it was possible to secure spheroidized cementite having an average aspect ratio of 2.5 or less only by spheroidizing heat treatment shorter than the conventional heat treatment of 30 hours or more.

7 and 8 are microstructure photographs taken with a scanning electron microscope (SEM) after spheroidizing the wire rods of Example 1 and Comparative Example 1 of the present invention, respectively.

Referring to FIGS. 7 and 8 , in Example 1, as compared with Comparative Example 1, spherical cementites are evenly distributed, and it can be seen that spheroidization is performed at a faster rate.

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. Accordingly, even after the softening heat treatment, the cementite average aspect ratio was 8.5, so 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 was 850 ° C., exceeding the Acm ° C transformation point or higher, and as the cooling time required until the end of the phase transformation became longer, the effect of refining the grains was greatly reduced. Accordingly, even after the softening heat treatment, the cementite average aspect ratio was 6.2, so 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, it satisfies the component range suggested by the present invention, but the Tpf - Tf value greatly exceeds 50° C. at 85° C., and the internal/external temperature deviation of the material during rolling greatly increases, so that the average grain size in the center is A coarse microstructure of 15 μm was derived. Accordingly, even after the softening heat treatment, the cementite average aspect ratio was 7.5, so a spheroidized structure could not be obtained, and the tensile strength value was derived as high as 810 MPa.

In the case of Comparative Example 4, although the component range suggested by the present invention was satisfied, the deformation amount was 0.32, which was significantly less than the critical deformation amount of 0.69. Accordingly, even after the softening heat treatment, the cementite average aspect ratio was 5.5, so that a spheroidized structure could not be obtained, and the tensile strength value was as high as 770 MPa.

As described above, according to the embodiment of the present invention, fine grain distribution was derived by controlling the alloy component and the manufacturing method. Accordingly, the spheroidizing heat treatment process involved for softening after manufacturing the wire rod can be shortened or omitted, so that the product price competitiveness can be secured.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art will not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (12)

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 (excluding 0), containing the remaining Fe and unavoidable impurities,
The prior austenite grain size of the microstructure is 3 to 10 μm,
A wire rod for bearings with a misorientation angle of 15° or more and the sum of the grain boundary lengths of 1,000 to 4,000 mm/mm ² per unit area. According to claim 1,
The microstructure is a wire rod for bearings composed of reticulated proeutectoid cementite at the grain boundary and pearlite in the grain. 3. The method of claim 2,
A wire rod for bearings with a layer spacing of 0.05 to 0.2 μm in pearlite. According to claim 1,
A wire rod for bearings with a tensile strength of 1,200 MPa or more and a reduction in cross-sectional area (RA) of 20% or more. According to claim 1,
A wire rod for bearings having an average aspect ratio of cementite of 2.5 or less after one-time softening heat treatment. According to claim 1,
A wire rod for bearings with a tensile strength of 750 MPa or less after one soft nitriding heat treatment. 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 (excluding 0), Heating the billet containing the remaining Fe and unavoidable impurities in a temperature range of 950 to 1,050 ℃;
manufacturing a wire rod by finish hot rolling in a temperature range of Ae1 to Acm°C with a deformation amount greater than or equal to the critical deformation amount expressed by the following formula (1); and
After cooling the wire rod to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, cooling at a rate of 1° C./sec or less; a method of manufacturing a bearing wire rod comprising a.
Equation (1): -1.6Ceq ² + 3.11Ceq - 0.48
(Here, Ceq = C + Mn/6 + Cr/5, and C, Mn, and Cr mean weight percent of each element.) 8. The method of claim 7,
The wire rod is a method of manufacturing a bearing wire that satisfies the following formula (2).
Equation (2): Tpf - Tf ≤ 50°C
(Here, Tpf is the average surface temperature of the wire rod before finishing hot rolling, and Tf is the average surface temperature of the wire rod after finishing hot rolling.) 8. The method of claim 7,
A method for manufacturing a bearing wire rod with a heating time of 90 minutes or less. 8. The method of claim 7,
A method of manufacturing a bearing wire rod having an average austenite grain size (AGS) of 5 to 20 μm before finish hot rolling. 8. The method of claim 7,
After cooling, a softening heat treatment step of heating the wire rod to Ae1 to Ae1+40° C. and maintaining it for 5 to 8 hours; 12. The method of claim 11,
After the softening heat treatment, cooling to 660° C. at a rate of 20° C./hr or less; Method for manufacturing a bearing wire rod further comprising a.

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