KR101359125B1 - Method for manufacturing wire rod for high carbon chromium bearing steel having improved fatigue life, method for manufacturing bearing steel using the wire rod and high carbon chromium bearing steel manufactured by the method - Google Patents

Abstract

By weight% C: 0.50-1.20%, Si: 0.15-2.00%, Mn: 0.05-0.45%, P: 0.025% or less, S: 0.025% or less, Cr: 0.10-1.60%, Ce: 0.01-0.30%, A high carbon fatigue life improved, comprising the steps of preparing a steel material consisting of the balance Fe and other unavoidable impurities, heating the steel at 900 to 1100 ℃ for 0.5 to 2 hours, and hot rolling at 700 to 1100 ℃ A method for producing a wire rod for chrome bearing steel, a method for manufacturing a bearing steel using the wire rod, and a bearing steel produced by the method are provided.
According to the present invention, it is possible to shorten or omit the time required for the bearing steel spheroidizing heat treatment process, and to provide a bearing steel having improved fatigue life by miniaturizing the cast structure and the pearlite lamellar structure after hot rolling.

Description

METHOD FOR MANUFACTURING WIRE ROD FOR HIGH CARBON CHROMIUM BEARING STEEL HAVING IMPROVED FATIGUE LIFE , METHOD FOR MANUFACTURING BEARING STEEL USING THE WIRE ROD AND HIGH CARBON CHROMIUM BEARING STEEL MANUFACTURED BY THE METHOD}

The present invention relates to a method for producing a wire rod for high carbon chromium bearing steel with improved fatigue life, a method for producing a bearing steel using the wire rod, and a high carbon chrome bearing steel produced by the method.

Since the spheroidization process in the manufacturing process of the bearing steel is a process that requires the growth of spherical particles, it takes a long time and in fact, heat treatment for 24 hours or more has been performed.

For the same reason, techniques for shortening and eliminating spheroidization heat treatment have been developed, and such techniques can be broadly divided into two types.

First, the billet is reheated above the austenite single phase region and then continuously rolled from A _cm to below the A ₁ vacancy transformation point to produce a spheroidized carbide seed, followed by the ultra-cooling pattern to seed the seed. ) How to grow. In addition, this rolling pattern is repeatedly applied to increase the number of carbide seeds.

Second, a method of actively adding high temperature carbide forming elements and growing them as spheroidized carbide seeds during cooling.

The above methods may cause problems such as roll breakage and high rolling load because low-temperature reverse rolling must be performed to secure sufficient spheroidized carbide seeds.

One aspect of the present invention is a method of manufacturing a wire rod for high carbon chromium bearing steel that can significantly reduce the spheroidization heat treatment time by miniaturizing the pearlite lamellar structure before the spheroidizing heat treatment, a method of manufacturing the bearing steel using the wire, and the fatigue produced by the method. High carbon chromium bearing steel with increased service life is presented.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention, by weight% C: 0.50 ~ 1.20%, Si: 0.15 ~ 2.00%, Mn: 0.05 ~ 0.45%, P: 0.025% or less, S: 0.025% Hereinafter, preparing a steel comprising Cr: 0.10 to 1.60%, Ce: 0.01 to 0.30%, balance Fe and other unavoidable impurities, heating the steel at 900 to 1100 ° C. for 0.5 to 2 hours, and 700 to 1100 It provides a method for producing a high carbon chromium bearing steel wire with improved fatigue life including the step of hot rolling at ℃.

Another aspect of the present invention, the step of cooling the wire after the spheroidizing heat treatment at 750 ~ 850 ℃ 0.5 ~ 2 hours, quenching step of quenching after heating 0.5 ~ 2 hours at 800 ~ 880 ℃, and 150 ~ 200 ℃ It provides a method for producing a high carbon chromium bearing steel with improved fatigue life including the step of tempering 0.5 to 2 hours.

Another aspect of the present invention provides a high carbon chromium bearing steel having improved fatigue life produced by the above method.

According to one aspect of the invention, the time required for the bearing steel spheroidizing heat treatment process can be shortened or omitted.

According to another aspect of the present invention, it is possible to provide a bearing steel with improved fatigue life by miniaturizing the cast structure and the pearlite lamellar structure after hot rolling.

1 is a photograph measuring the pearlite lamellar spacing according to a comparative example of the present invention.
Figure 2 is a photograph of measuring the pearlite lamellar spacing according to an embodiment of the present invention.
3 is a microstructure photograph after spherical heat treatment according to a comparative example of the present invention.
Figure 4 is a microstructure photograph after the spheroidization heat treatment according to an embodiment of the present invention.
5 is a graph measuring B ₁₀ fatigue life according to an embodiment of the present invention and a comparative example.

Hereinafter, a method for manufacturing a high carbon chromium bearing steel having improved fatigue life and a high carbon chromium bearing steel produced by the method so that a person skilled in the art can easily carry out the present invention. To explain.

In general, the bearing steel is refined while maintaining a strong reducing atmosphere in the ladle after the steelmaking in the converter or the electric furnace to reduce the amount of non-metallic inclusions, and after the vacuum degassing process to reduce the oxygen content to 12 ppm or less, after After solidifying with cast iron or steel ingot in the casting process, it is subjected to crack diffusion treatment to remove segregation and macrocarbide in the center of the material and then rolled into bilette. After that, the rolling mill performs ultra-cold operation to soften the material, and is produced as a wire rod or rod of bearing steel, and the produced material is processed into ball, roller, or inner ring which are the rolling elements of the bearing through spheroidizing annealing. Subsequently, after hardening and tempering as hardening heat treatment, they are polished to produce a final product as a bearing.

The spheroidization heat treatment process in the above manufacturing process is mainly performed by diffusion at high temperature, and mainly starts at the defect or end portion of cementite in lamellar, and is caused by the difference in curvature between the end portion and the flat surface of the side. It is known that the cementite in the lamellar is segmented by the carbon concentration gradient, and then it is spheroidized to reduce the surface energy. The spherical particles thus formed are grown through a process similar to Ostwald ripening to form globular tissue. This spheroidization process is mainly observed directly under the vacancy transformation temperature, and the part existing as ferrite in the initial tissue remains in the form of ferrite, but the part existing as pearlite is changed into ferrite and spherical cementite, and thus the whole microstructure Consists of ferrite and spherical cementite. However, since the spheroidization process requires the growth of spherical particles, it takes a long time and in fact, heat treatment for 24 hours or more is performed.

The present invention intends to focus on a new method different from the conventional method in order to shorten and omit the nodular heat treatment, and to propose a method capable of significantly shortening the nodular heat treatment time by miniaturizing the pearlite lamellar tissue before the nodular heat treatment.

To this end, one aspect of the present invention, by weight% C: 0.50 ~ 1.20%, Si: 0.15 ~ 2.00%, Mn: 0.05 ~ 0.45%, P: 0.025% or less, S: 0.025% or less, Cr: 0.10 ~ 1.60%, Ce: 0.01 ~ 0.30%, balance Fe and other unavoidable impurity preparing a steel, heating the steel at 900 ~ 1100 ℃ 0.5 ~ 2 hours, and hot rolling at 700 ~ 1100 ℃ It provides a method for producing a high carbon chromium bearing steel wire with improved fatigue life.

In steel heating temperature and heating time, if the temperature is lower than 900 ℃, the load of the rolling roll is too large, which makes the rolling difficult and there is a risk of formation of cornerstone cementite. There is. In addition, if the heating time is shorter than 0.5 hours, the temperature inside the material is not uniform, and the rolling property is lowered. If the heating time is longer than 2 hours, the grains coarsen and the pearlite lamellar spacing may increase.

In the hot rolling temperature, when the temperature is lower than 700 ° C, the load of the rolling roll becomes large, so that rolling is impossible, and 1100 ° C cannot be higher than this because it is the maximum heating temperature.

The occurrence of central segregation during the casting of bearing steel is recognized as an inevitable phenomenon, but if it can be reduced, there is a significant advantage in terms of manufacturing cost since the load of the cracking process, which is a post process, is reduced. The central segregation during casting can be reduced by forming a large amount of fine equiaxed crystals in the center of the casting material, which can be reduced by inoculation. In other words, when an inoculant that promotes heterogeneous nucleation is added, certain components in the dissolved inoculum rapidly form oxides or precipitates with a small coagulation phase and lattice mismatch, and these oxides or precipitates minimize the increase of interfacial energy at the solid-liquid interface. As a result, fine isotropic crystals are formed by promoting heterogeneous nucleation. Since the bearing steel is a high carbon steel and solidification starts with austenite during casting, the inoculant for promoting the formation of equiaxed crystals requires austenite and oxides or precipitates with small lattice mismatch. Such inoculants are known to be AlCeO ₃ , CeO ₂ , Ce ₂ O ₃ , Ce ₂ O ₂ S, CeS, Ce ₂ S ₃ , TiC, TiN, TiO ₂ , Al ₂ O ₃ , but Al and Ti inclusions and sulfides can adversely affect the fatigue life of bearing steel. Therefore, more preferred inoculant may be referred to as CeO ₂ or Ce ₂ O ₃ . When the cast structure is refined by the input of Ce alloy, the pearlite lamellar structure after the hot rolling, which is a post-process, is also refined, thereby shortening the nodular heat treatment time after the hot rolling and miniaturizing the remaining spheroidized carbide. To this end, in the present invention, Ce alloy was added during steel refining.

The reason for limiting the numerical values of the above components will be described as follows. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

C: 0.50 to 1.20 wt%

Carbon is a very important element to secure the bearing strength. If the content of carbon is low, the strength and fatigue strength of the bearing is low, so it is not suitable as a bearing part, the content of carbon is preferably 0.50% by weight or more. On the other hand, if the carbon content is too high, the undissolved macrocarbide not only lowers the fatigue strength but also degrades the workability before quenching, so the upper limit of the carbon content is limited to 1.20% by weight.

Si : 0.15 ~ 2.00 wt%

When the content of silicon is low, it may be a problem of hardenability, so it is preferable to add 0.15% by weight or more. However, when the silicon content is too high, decarburization may occur due to the competition reaction with carbon, and like C, the workability before quenching decreases and the central segregation increases, so the silicon content is 2.00 wt%. It is the upper limit.

Mn : 0.05 ~ 0.45 wt%

Manganese is an important element in securing strength by improving the hardenability of steel. Therefore, it is preferable that Mn is contained 0.05 weight% or more. However, when the content of manganese is too high, the workability before quenching not only decreases, but also increases the precipitation of MnS, which adversely affects the center segregation and fatigue life, so that the content of manganese is 0.45% by weight or less.

P: 0.025 wt% or less

P is an element which segregates at grain boundaries and reduces the toughness of steel materials. Therefore, it is more preferable to actively limit the content. Therefore, when considering the load of the steelmaking process, it is preferable to limit the content to 0.025% by weight or less.

S: 0.025 wt% or less

S acts to increase the machinability of steel, but it is preferable to limit its content since it adversely affects not only segregation at grain boundaries but also toughness as in phosphorus, but also the fatigue life by combining with Mn to form an emulsion. Therefore, when considering the load of the steelmaking process, it is preferable to make the content less than 0.025% by weight.

Cr : 0.10 ~ 1.60 wt%

Cr is an element effective in improving the hardenability of the steel to impart hardenability and is effective in miniaturizing the structure of the steel. Therefore, it is preferable to add Cr at least 0.10% by weight. However, when the content of Cr is excessive, the effect is saturated, so the content of chromium is 1.60 wt% or less. Therefore, the content of Cr is preferably limited to 0.10 ~ 1.60% by weight.

Ce : 0.01 ~ 0.30 wt%

Ce is added to the inoculant and is an effective element to refine the structure of the steel, so it is preferable to add 0.01 wt% or more. However, when the content of Ce is excessively high, the stability of the steelmaking process is considerably lowered and oxide formation proceeds rapidly, so that the effect of promoting equiaxed crystal formation is saturated, so that the content of cerium is 0.30% by weight or less.

In an exemplary embodiment, the lamellar spacing of the wire rod after the hot rolling may be 50 to 230 μm, but is not limited thereto.

If the lamellar spacing is smaller than 50 mu m, the spherical carbide becomes too small, and if it is larger than 230 mu m, the number of spherical carbides becomes too small.

Another aspect of the present invention, the step of cooling the wire after the spheroidizing heat treatment at 750 ~ 850 ℃ 0.5 ~ 2 hours, quenching step of quenching after heating 0.5 ~ 2 hours at 800 ~ 880 ℃, and 150 ~ 200 ℃ It provides a method for producing a high carbon chromium bearing steel with improved fatigue life including the step of tempering 0.5 to 2 hours.

For the nodularization temperature and nodularization time, if the nodularization temperature is less than 750 ° C. or the nodularization time is shorter than 0.5 hour, there is a risk that the carbide will be spheroidized too long and if it is higher than 850 ° C. or longer than 2 hours, it will be completely dissolved.

In the heating temperature and time for quenching, if the quench structure is less than 800 ° C. or shorter than 0.5 hours, there is a risk that all of the spherical carbides are dissolved if it is higher than 880 ° C. or more than 2 hours.

For tempering temperatures and times, less than 150 ° C. or shorter than 0.5 hours results in less toughness recovery and more than 200 ° C. or longer than 2 hours, the hardness decreases drastically.

In an exemplary embodiment, the size of the residual carbide after the spheroidization heat treatment may be 0.1 to 0.5 μm, but is not limited thereto.

If the size of the residual carbide after spheroidization heat treatment is less than 0.1 µm, the contribution to the improvement of wear resistance is insufficient. If the size of the carbide is larger than 0.5 µm, the contribution to the improvement of wear resistance is not only saturated but too large.

In an exemplary embodiment, the number of residual carbides after the spheroidization heat treatment may be 1.8 × 10 ⁶ to 3.0 × 10 ⁶ particles / mm 2, but is not limited thereto.

When the number of residual carbides after spheroidization heat treatment is less than 1.8 × 10 ⁶ pieces / mm 2, the contribution to wear resistance improvement is insufficient, and when more than 3.0 × 10 ⁶ pieces / mm 2, the wear resistance improvement contribution is saturated.

Another aspect of the present invention provides a high carbon chromium bearing steel having improved fatigue life produced by the above method.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

After manufacturing the bearing steel molten steel was added to the ferroalloy during refining to prepare a steel having a component as shown in Table 1 below. The content unit of each component is% by weight.

division C Si Mn P S Cr Ce Comparative steel 0.99 0.25 0.34 0.009 0.008 1.47 - Invention river 1.01 0.23 0.33 0.011 0.008 1.42 0.183

The prepared steel was heated at 1050 ℃ for 90 minutes and then hot rolled to prepare a 30 mmf wire. 1 shows the pearlite lamellae structure after hot rolling of the comparative steel, and FIG. 2 shows the pearlite lamellae structure after the hot rolling of the inventive steel. In FIG. 1, the pearlite lamellar spacing of the comparative steel was measured to be 238 μm, but in the case of the inventive steel of FIG. 2, it was 184 μm. Therefore, it can be confirmed that the inventive steel incorporating the Ce alloy iron prepared according to the present invention had a much finer pearlite lamellar structure after hot rolling compared to the comparative steel.

After hot rolling, spheroidization heat treatment was performed, and Table 2 shows the spheroidization conditions of the comparative steel and the inventive steel and the size and number of spheroidal carbides remaining before and after spheroidization heat treatment.

division Lamellar Lamellar Spacing after Hot Rolling (㎛) Nodular heat treatment condition Carbide features after spheroidization Temperature (℃) Time (hr) Average size (㎛) Number (pcs / ㎡) Comparative steel 238 790 6 0.87 1.85 × 0 ⁶ Invention river 184 790 2 0.41 2.81 × 0 ⁶

The average size of spherical carbide was 0.87 μm and the number was 1.85 × 10 ⁶ pieces / mm2 as a result of spheroidization of spherical carbide at 790 ℃ for 6 hr. It was very fine (0.41 ㎛) and the number was much more 2.81 × 10 ⁶ / mm2. This result is also confirmed in the microstructure after the spheroidizing heat treatment of the comparative steel and the inventive steel shown in FIGS. 3 and 4.

After spheroidizing heat treatment, quenching and tempering were performed to give final properties, and then fatigue life was evaluated by performing a thrust-type rolling fatigue test. As the test conditions, the contact stress was 5,836 MPa and the rotation speed was 1,000 rpm. 24 pieces were tested per steel grade to obtain the B ₁₀ life. Figure 5 shows the fatigue life of the comparative steel and the inventive steel, the B ₁₀ life of the comparative steel is 3.45 × 10 ⁷ while the invention steel is 1.22 × 10 ⁸ has the effect of increasing the fatigue life by more than 3.5 times.

From this, bearing steel with reduced segregation by adding Ce can significantly reduce spheroidization time as the cast structure and the pearlite lamellar structure after hot rolling become finer, and the size of spherical carbides is further refined and the number of spherical carbides is increased, resulting in fatigue life. It is expected to be able to improve.

Claims (6)

By weight% C: 0.50-1.20%, Si: 0.15-2.00%, Mn: 0.05-0.45%, P: 0.025% or less, S: 0.025% or less, Cr: 0.10-1.60%, Ce: 0.01-0.30%, Preparing a steel material comprising residual Fe and other unavoidable impurities;
Heating the steel at 900 to 1100 ° C. for 0.5 to 2 hours; And
Method for producing a high carbon chromium bearing steel wire with improved fatigue life, including the step of hot rolling at 700 ~ 1100 ℃. The method of claim 1,
A method for producing a high carbon chromium bearing steel wire having improved fatigue life, wherein the lamellar spacing of the wire rod after the hot rolling is 50 to 230 μm. Cooling the wires according to claim 1 or 2 after spheroidizing heat treatment at 750 to 850 ° C. for 0.5 to 2 hours;
Quenching step of quenching after heating at 800 ~ 880 ℃ 0.5 ~ 2 hours; And
Method for producing a high carbon chromium bearing steel with improved fatigue life, comprising the step of tempering 0.5 ~ 2 hours at 150 ~ 200 ℃. The method of claim 3, wherein
A method for producing a high carbon chromium bearing steel having improved fatigue life, wherein the size of residual carbide after the spheroidizing heat treatment is 0.1 to 0.5 µm. The method of claim 3, wherein
A method for producing a high carbon chromium bearing steel having improved fatigue life, wherein the number of residual carbides after the spheroidizing heat treatment is 1.8 × 10 ⁶ to 3.0 × 10 ⁶ holes / mm 2. A high carbon chromium bearing steel with improved fatigue life prepared by the method of claim 3.

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