JP2006200027A - High-carbon chromium steel for bearing and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-carbon chromium steel for bearing superior in machinability and rolling fatigue life, and to provide a production method therefor. <P>SOLUTION: The high-carbon chromium steel for bearing has a chemical composition which satisfies JIS G 4805 and also satisfies 0.0009 mass% or less O, 0.005 mass% or less Al and 0.005 mass% or less S; and includes oxides among which the number of oxides with sizes of 3 μm or larger existing in a cross section of 160 mm<SP>2</SP>in parallel to a rolling direction when observed by a microscope is 100 or less and the number of oxides with sizes of 10 μm or larger is 2 or less, and among which the total number of alumina-based and spinel-based oxides is less than 60% of the total oxide number, when their composition ratios are further referred to. The method for producing the high-carbon chromium steel for bearing comprises a deoxidizing step of using a deoxidizer which does not substantially contain Al; and a subsequent ladle-refining step of controlling basicity ((CaO)%/(SiO<SB>2</SB>)%) in slag to less than 0.8 to 3.0. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an improvement in bearing steel for processing into element members of rolling bearings such as ball bearings and roller bearings, and by controlling the size and number of trace amounts of oxides in the steel and the composition thereof. The present invention relates to a steel and a method for manufacturing the same that greatly improve the tool life and rolling fatigue life as a bearing at the same time.

  A high carbon chromium bearing steel specified in JIS G 4805 is often used for rolling bearings. This so-called bearing steel is the largest material on the material side that controls the rolling fatigue life, which is the performance in use. It is said that the factor is a very small amount of hard oxide inclusions that cannot be completely removed after deoxidation, and that is why bearing steel manufacturers are in the process of thoroughly removing oxide inclusions. In recent years, the oxygen content (hereinafter simply referred to as O), which is considered to be an index representing the total amount of oxide inclusions contained in steel, is 0.0009 mass% or less (when this mass ratio is expressed in ppm). The bearing steel produced in Japan is an extremely clean bearing steel, which is single-digit because it is a single digit, and all the percentages in the following steels or percentages representing slag components mean mass%) Is of normal quality.

  The standard manufacturing method of high clean bearing steel in Japan is summarized in Non-Patent Document 1, but in the description there, it is clearly shown that the present invention is fundamentally different from the background art. The contents are written as follows.

  First, p160 describes “bearing steel that is fully deoxidized steel by Al” as to what is achieved by deoxidation of high-clean bearing steel in Japan. As the component value, p164 Table 5 lists the Al amount of 0.010%. That is, it is understood that the current high clean bearing steel in Japan can achieve its low O value by completely deoxidizing by adding about 0.010% of Al which is a strong deoxidizer. The

Next, regarding the slag composition in the reduction smelting step, the lower the slag basicity (CaO)% / (SiO ₂ )%, the lower the concentration of lower oxides (FeO and MnO) in the slag and the lower the O in steel. 15 and 16 on p165, and the overall data of domestic companies is that if the (CaO)% / (SiO ₂ )% is 3 or more, the O concentration in steel is similar. A single ppm is shown in FIG. 19 on p167. These descriptions show that the low O value of current high cleanliness bearing steel is deoxidized with strong Al, and then the basicity of the slag is selected to a high value of 3 or more in the reduction scouring process, so that the reoxidation property in the slag It is understood that this is achieved by reducing the components.

  However, the high-clean bearing steel manufactured under the above-described manufacturing conditions does not contain O in the molten steel even if the amount of oxide inclusions finally remaining in the steel is an extremely small amount of single ppm in the amount of O contained. Since the strongest Al is used as a deoxidizer for removal, extremely hard oxide inclusions are produced as follows.

That is, when Al is added for deoxidation after oxidative refining, O in the steel first forms alumina (Al ₂ O ₃ ), and this alumina is used for the slag line of the ladle in the next ladle refining. Most of it reacts with Mg dissociated and eluted from MgO-C brick to become spinel (Al ₂ O ₃ .MgO), and a small amount remains as alumina.

Most of the spinel (Al ₂ O ₃ .MgO) system that can be formed in this way (the spinel system is the same in crystal structure even if the composition is shifted from the stoichiometric spinel (Al ₂ O ₃ .MgO)). And alumina (Al ₂ O ₃ ) system (alumina system is the same physical as alumina with small amounts of other components but not pure alumina). Oxide (referred to as a component range indicating characteristics) (hereinafter referred to as “spinel”, etc., unless otherwise specified) is extremely hard, although its residual amount is extremely small. However, there were problems in the processing steps and performance problems as product bearings as listed below.

  The first problem that occurs because the oxide in the bearing steel is extremely hard, such as spinel, is that when the part is a bearing race, it is processed into a shape close to the final product by cutting. However, tool wear is fast in the cutting process. In the machining of bearings, the cost of the cutting process is a relatively large part of the total cost, as in the case of many other machine parts, and bearing manufacturers want a bearing steel that can be easily cut. It was.

  The second problem due to the extremely hard oxide in steel, that is, the performance problem of the product bearing, is that the rolling fatigue life of the bearing is short for the very small amount of oxide in question. The reason is that cracks tend to occur due to strong stress concentration around inclusions such as a hard spinel system each time the ball passes over the race.

  Recognizing that the oxide inclusions remaining in a trace amount as described above are extremely hard spinel, etc., there is a difficulty in the machining process and product bearing performance, and in order to avoid it, the composition of the oxide Several attempts have been reported so far to try to change to a non-rigid or similar.

  For example, Non-Patent Document 2 reports a method in which Al is not used for deoxidation after oxidative refining with the aim of not making the oxide composition hard, but in that report it is the most powerful deoxidizer. As a result of not using certain Al, the O content in the steel remains in the range of 0.0008% to 0.0015%, and high cleanliness bearing steel cannot be stably produced.

  In Patent Document 1, as a method of thoroughly reducing the amount of non-metallic inclusions in order to improve the rolling fatigue life of product bearings, reduction scouring is performed in the presence of high basicity slag, and Al is not used as much as possible. Discloses a high cleanliness bearing steel in which O is 0.0006% or less by a vacuum degassing smelting method and a method for producing the same, but this technology is insufficient to change the oxide composition to a non-hard one. For this reason, although the rolling fatigue life of the product bearing has been improved, the machinability has only been maintained at the same level as conventional steel, and both of them have not been improved.

Japanese Patent Publication No. 4-5742 Nishiyama Memorial Technology Lecture Text “Bearing Steel Cleaning” Held on October 22, 2004 by the Japan Steel Association “Stahl und Eisen” 117 (1997) Nr. 8, p79-89

  The present invention is to solve the problem of compatibility between the machinability and the rolling fatigue life of the bearing steel having the background as described above, while significantly reducing O and Al in the steel, and also in the hard oxide in the steel. High carbon chromium bearing steel with a reduced proportion of system inclusions and its manufacturing method are not prone to harmful side effects such as adding some elements, but are produced by deoxidation after oxidative refining. The oxide inclusions that cannot be completely removed are selected appropriately for the element to be used as a deoxidizer so as not to be hard, which is disadvantageous to the rolling fatigue life and tool life during cutting. The present invention also provides a method for producing clean steel by appropriately adjusting the slag composition so that the oxide inclusions that are sequentially removed from the molten steel are not changed to hard ones in ladle smelting.

  The inventors of the present invention do not significantly improve the rolling fatigue life and machinability simply by reducing O and Al in the steel, and further, the size and number of oxide inclusions and the composition of the oxide inclusions by new means. The inventors have found that softening control is necessary, and have completed the present invention.

That is, the gist of the invention is:
(1) The steel is composed of steel that satisfies JIS G 4805, O: 0.0009% or less, Al: 0.005% or less, and S: 0.005% or less. The number of oxides having a size of 3 μm or more is 100 or less in a viewing field area of 160 mm ² and 2 or less of those having a size of 10 μm or more. A high carbon chromium bearing steel characterized in that the total number with the system is less than 60% of the total number of oxides.
Alumina system: (MgO) and (SiO ₂ ) are less than 3%, and (CaO) and (CaO) / ((CaO) + (Al ₂ O ₃ )) are not more than 0.08.
Spinel system: (MgO) in the range of 3% to 20% and binary system with the balance being (Al ₂ O ₃ ) contain 15% (CaO) and / or 15% (SiO ₂ ). It may have a spinel crystal structure.
(2) Production of high carbon chromium bearing steel in which the chemical composition of the steel satisfies JIS G 4805, and satisfies O: 0.0009% or less, Al: 0.005% or less, and S: 0.005% or less. A method of using a deoxidizer containing substantially no Al for deoxidation after oxidation refining by a converter or an electric furnace and subsequent component adjustment, basicity of slag in the next ladle scouring ((CaO)) % / (SiO ₂ )%) of 0.8 or more and less than 3.0, followed by a vacuum degassing treatment step of 35 minutes or more. Production method.

  The bearing steel of the present invention greatly improves the tool life in high-speed cutting without using coolant with a carbide tool in recent years, and is suitable for long-term stable operation as a product bearing, so the industrial utility value is very large .

  Further, the effect obtained by the present invention is not affected by the slight fluctuations in the contents of Si, Mn, Cr, and Mo. Therefore, not only the high carbon chrome bearing steel type 2 SUJ2 shown in the examples but also other The same effects can be obtained including the first to fifth types.

  In addition, the reason why machinability and rolling fatigue life could be improved is that the hardness of the bearing steel that has been hardened and hardened by modifying a very hard oxide in the steel is approached. Therefore, it can be said that the heterogeneity with respect to the oxide fabric in the bearing steel fabric has weakened. As a result, it is expected that the rubbing noise generated when the race and ball rub against each other each time the bearing rotates will be greatly reduced, so it is used for air conditioners that are continuously operated in quiet environments such as acoustic equipment and hospitals. It is considered suitable for bearings.

  First, the chemical composition of the bearing steel of the present invention, the size and number of oxides contained therein, and the reasons for limiting the composition and ratio thereof will be described.

  First, in the high carbon chromium bearing steel of the present invention, the chemical composition of the steel satisfies JIS G 4805. Chemical constituents that satisfy JIS G 4805, except for S, page 1111 of “JIS Handbook (1) Steel I” (Japan Standards Association, 31st January 2004, first edition, first edition) Of “High Carbon Chromium Bearing Steel” in “Table 2 Chemical Components” and the components listed in the remarks column below the table. In Table 2, five types of SUJ1 to 5 are defined. For example, in SUJ2 (unit%), C: 0.95 to 1.10, Si: 0.15 to 0.35, Mn: 0.50 Hereinafter, P: 0.025 or less, S: 0.025 or less, and Cr: 1.30 to 1.60.

O: 9 ppm or less O remains in the steel and basically forms harmful oxides, so it is preferably as small as possible, preferably 6 ppm or less. However, if it is 9 ppm or less that can be easily obtained by ordinary oxidation refining → ladle refining → vacuum degassing step, the effect of the present invention can be obtained, so the content is 9 ppm or less.

Al: 0.005% or less Since Al makes the oxide in the steel hard, it is desirable that it be as little as possible, preferably 0.002% or less. However, if Al is not used as a deoxidizer, the effect of the present invention can be obtained if it is 0.005% or less, which can be easily achieved. Therefore, the content is made 0.005% or less.

S: 0.005% or less S is a starting point of fatigue failure in the case where O is single ppm as in the case of bearing steel in recent years. Should be as low as possible. Further, in the case of high-speed dry cutting with a cemented carbide tool targeted by the present invention, the S content has little influence on the tool life, so it is possible to reduce S from that point as well. However, the ladle scouring with the low basicity slag according to the present invention is less difficult to remove S from the molten steel than the case of the high basicity slag which is the background art, and is sufficient from the viewpoint of rolling fatigue life and according to the present invention. S as 0.005% or less as a component range that can be easily achieved by the production method. However, 0.002% or less is even better.

Size and number of oxides The size and number of oxides observed in 160 mm ² of the cross section parallel to the rolling direction are 100 or less, and two of which are 10 μm or more, with a circle equivalent diameter of 3 μm or more. The following. Here, the equivalent circle diameter is calculated by “√ (major axis × minor axis)”. In addition, the oxide referred to here is calculated and counted by the contour of the oxide in the composite state even when it is combined with sulfide as well as those existing alone. In this way, the upper limit of the number of observations by size of oxide is defined by the fact that even if the harmfulness is improved by softening the hardest oxide that is most harmful to the rolling fatigue life, it is still a large oxide. Because it is harmful. What is really detrimental to the rolling fatigue life is the largest oxide present at the site where large contact stress is generated when it becomes a bearing, so the above 10 μm or more corresponds almost directly, but 3 μm Since the number of detections is large and the number of detections is 10 μm or more, there is no chance of hitting them.

Composition of oxide As the composition of oxide having a particle size of 3 μm or more as observed in the previous section, the total number of alumina and spinel systems should be less than 60% of the total number of oxides. The reason why the composition ratios of the oxides are defined in this way is to define the ratio at which the softening modification of the hard oxide is achieved, which is extremely harmful to both the rolling fatigue life and the tool life during cutting. is there. In the background technology for producing bearing steel with high cleanliness, the oxides that finally remain in the steel are mainly hard spinel (Al ₂ O ₃ .MgO) because Al is used as a deoxidizer. It becomes. In the method according to the present invention, even if Al is not used as a deoxidizer, the oxide composition tends to be a spinel system or the like due to some Al contained in the additive alloy or Al coming from a refractory in contact with molten steel. Therefore, the slag composition at the time of ladle scouring was appropriately selected, and the oxide composition was induced to something other than a spinel system. In the method according to the present invention, the number ratio of extremely hard spinel oxides and the like can be reduced to less than 60%, and both the tool life and rolling fatigue life at the time of cutting are compared with the background art. Therefore, the composition ratio of the oxide composition is defined as described above.

  Next, the manufacturing method of the high carbon chromium bearing steel of the present invention will be sequentially described in the following (1) to (6).

  (1) The first essential point of the present invention is that, in deoxidation after oxidation refining of bearing steel in a converter or electric furnace, and addition of an alloy for adjusting the components thereafter, an extremely hard spinel system as a deoxidation product, etc. In order to avoid this, it is possible to deoxidize and adjust the components with Fe-Si or metal Si which does not substantially contain Al, without using Al having a strong deoxidizing power. Here, the deoxidizer which does not substantially contain Al refers to metal Si and, in the case of Fe—Si, one having a ratio of the Al content to the Si content of 0.03 or less.

  (2) Even in the case of bearing steel, O in the steel can be reduced to a single ppm even if it is deoxidized with Si, which has a weaker deoxidizing power than Al. The reason for this is that, in the case of bearing steel, the C content is extremely high at about 1%, so that in the “vacuum degassing process step” that is performed subsequent to or simultaneously with the reduction smelting step, the vacuum degassing is better. This is because it can be acidified, and it does not have to depend only on a deoxidizing agent such as Al or Si. That is, in this “vacuum degassing process”, a reaction of C + O → CO ↑ occurs between O and C in the molten steel, but this reaction proceeds to the right if C on the left side is high, that is, Since the deoxidation reaction proceeds further, and the molten steel is exposed to a high vacuum atmosphere in order to degas the target molten steel by vacuum, the equilibrium of the reaction proceeds to the right side on the gas generation side. This is because the deoxidation reaction proceeds well.

  (3) That is, even if deoxidizing with Si with weak deoxidizing power, single ppm sufficiently clean low O steel can be obtained. However, in the conventional manufacturing technology of clean bearing steel, deoxidizing with Si However, the ratio of hard spinel oxides could not be lowered sufficiently. The reason is that, as described in the background art, the basicity of slag in the reduction scouring process was increased to 3.0 or more in order to obtain low O steel.

(4) When the same high basicity slag as in the conventional case is used in the reduction smelting process, the equilibrium oxygen concentration as the multi-component slag composition is extremely low, which is a strong deoxidation product of a deoxidizing element. intensified prone to decomposition reaction of Al ₂ O ₃ → 2Al + 3O to alumina, even though not used Al as a deoxidizer in a vacuum degassing process, a component such as a lining brick ladle Al ₂ O ₃ and further Al returns slightly from the Al ₂ O ₃ in the slag to the molten steel. As a result, the Al that has returned to the steel proceeds to the next casting process, and as the molten steel temperature decreases toward the solidification temperature, the deoxidation reaction further proceeds, and in the casting process around the tundish and around the mold. When molten steel comes into contact with some kind of reoxidation source, it forms an extremely hard oxide such as a spinel system. The target "do not make hard oxide" is not fully achieved.

  (5) Therefore, Al does not return to the molten steel while maintaining a low O value by “adjusting the slag composition in the reduction smelting to a lower basicity side than before”, which is the second essential point of the present invention. As a result, the deoxidation product newly generated after the casting process does not become a hard spinel system, etc., and both the tool life during cutting and the rolling fatigue life as a product bearing have been greatly improved at the same time. Is. In order to lower the slag basicity, light calcined dolomite or quartzite may be increased by reducing quick lime in the raw material. When the slag basicity is lowered, Al remaining in the molten steel can be reduced. However, since the tendency not to remove O and S in the molten steel is increased, it is not preferable to reduce the basicity too much. Therefore, the lower limit of the basicity is set to 0.8 or more from the viewpoint of the deoxidizing ability and the desulfurizing ability of the slag, and the upper limit of the basicity for obtaining the softening effect of the oxide is set to less than 3.0.

  (6) Finally, the third essential point of the present invention is that the molten steel is sufficiently circulated by the vacuum degassing treatment step in which the vacuum degassing treatment time is 35 minutes or more, and the oxide suspended and suspended in the steel is floated as much as possible. It is separated and absorbed in the top slag. As the vacuum degassing process, RH vacuum degassing is preferably used. In other vacuum degassing process steps that do not depend on RH, the same effect can be obtained by sufficiently securing the processing time according to the performance.

  Below, the implementation conditions of the present invention and the effects obtained there will be described in detail. In addition, the underline attached | subjected to following Table 1, 2, 4 shows that it remove | deviates from the scope of the present invention.

  The bearing steel targeted by the present invention is a high carbon chrome bearing steel defined in JIS G 4805. As an example of the bearing steel, the present invention is applied in a converter-LF-RH-CC (continuous casting) process. And a method using only a part of the present invention and a method according to the background art produce high-clean bearing steels having the chemical components shown in Table 1, and after a predetermined soaking diffusion treatment, hot rolling is performed to 162 mm square. Billet.

  For the machinability test and the rolling fatigue test, the billet was further rolled into a 65 mmφ bar steel, and then annealed for spheroidizing carbides to obtain a test steel.

  For the machinability test, the above test steel was used as it was, and the cutting test conditions were a cutting speed of 200 m / min, a cutting depth of 2 mm, a feed rate of 0.25 mm / rev, and a JIS-P20 equivalent cemented carbide with TiN. Using a tool coated with the above-mentioned coating, the outer periphery was turned without coolant, and the wear of the tool was measured every 5 minutes. The time when the flank wear width reached 0.2 mm was determined as the tool life.

  For the rolling fatigue life evaluation test, a plurality of disk-shaped test pieces were used, which were roughly cut from the material of the above test steel and machined after normal quenching and low-temperature tempering heat treatment. Using the Mori-type thrust rolling fatigue tester, measure the number of loadings of each test piece until flaking occurs, and 10% of the number of the test specimens will have a 10% failure life that causes fatigue failure. Asked by paper.

  Table 2 shows the slag composition sampled at the end of the reductive smelting step with deoxidizer and LF after oxidative refining, which characterizes the difference between the present invention and the background art, and the vacuum degassing time with RH. Table 3 shows the composition of the deoxidizer used.

  Now, in the present invention, when clean bearing steel is manufactured by the above-described manufacturing method, the cause of greatly improving the machinability and rolling fatigue life was investigated from various factors of the steel material quality. It has been found that the above two properties are greatly improved at the same time when specific conditions are established in the size and number of the objects and their composition.

First, regarding the size and number of oxides, a microscopic sample was prepared from a cross section parallel to the rolling direction at a ½ radius portion of a 65 mmφ steel bar, and all oxides having an equivalent circle diameter of 3 μm or more found in a measurement field of view 160 mm ² were obtained. It is counted that the number is 100 or less, and that the number of particles having a particle size of 10 μm or more is 2 or less. The upper equivalent circle diameter is calculated by √ (major axis × minor axis).

  Here, the determination method of the oxide by an optical microscope can be easily determined because the oxide is darker and darker than the sulfide, and the Ti carbonitride often observed is a unique pink color. . In addition, when the oxide is simply combined such as sulfide or Ti carbonitride adjoining or surrounding the oxide, the size is calculated based on the outline of the oxide only and counted. If oxides and other compositions are complexly combined, calculate the total oxide size once and measure the composition by energy dispersive X-ray spectroscopy in the next step. Based on the result, a determination is made as to whether it is mainly included as an oxide or excluded mainly as inclusions of other compositions.

Next, as a method for investigating the oxide composition, the total number of oxides having a size of 3 μm or more observed as described above was measured by energy dispersive X-ray spectroscopy, and the composition was determined as (Al ₂ O ₃ ) When converted to a quaternary system of-(CaO)-(MgO)-(SiO ₂ ), it was found that almost all oxides can be classified into the following four types of component systems.
First classification: (MgO) and (SiO ₂ ) are both less than 3% and (CaO) is a ratio of (CaO) / ((CaO) + (Al ₂ O ₃ )) of 0.08 or less. Mostly (Al ₂ O ₃ ). Therefore, this is called an alumina system.
Second classification: In many cases, the binary system is (MgO) in the range of 3% to 20% and the balance is (Al ₂ O ₃ ), but within about 15% (CaO) and / or within about 15% (SiO ₂ ) is often mixed. Since the crystal structure is called spinel, this composition is called a spinel system.
Third classification: (MgO) and (SiO ₂ ) are less than 3%, and (CaO) exceeds 0.08 in the ratio of (CaO) / ((CaO) + (Al ₂ O ₃ )). This ratio may reach 0.50. Since it is close to the binary system of (CaO)-(Al ₂ O ₃ ), it is called a calcium aluminate system.
Fourth classification: (MgO) is hardly contained, and (CaO)-(Al ₂ O ₃ ) binary system contains various amounts of (SiO ₂ ) of 3% or more and up to about 50%. Here, the acronym for each component is taken and called the C-A-S ternary system.

  Table 4 summarizes the size and number of oxides of the bearing steel and the comparative material according to the present invention and the composition ratio when they are classified according to the above four classifications, and the tool life and the tool life in the peripheral turning test conducted under the test conditions. It shows the rolling fatigue life. Here, one oxide may be composed of two or more phases, and each phase may be classified into the above four classifications. In such a case, they are assigned to the classification having the largest area ratio.

From the results in Table 3, Fe-Si or metal Si containing no substantial Al was used for deoxidation after oxidation refining and subsequent component adjustment, and (CaO)% / as slag composition in reductive refining with LF Only when (SiO ₂ )% is selected from 0.8 to less than 3.0 and the vacuum degassing time in RH is set to 35 minutes or more, the number of oxides having a size of 3 μm or more can be suppressed. In addition, it is obvious that the composition can be prevented from becoming only a hard material such as a spinel system, and the tool life and rolling fatigue life in cutting can be greatly improved at the same time.

Claims (2)

The steel chemical composition satisfies JIS G 4805, and is made of steel that satisfies O: 0.0009 mass% or less, Al: 0.005 mass% or less, and S: 0.005 mass% or less in the rolling direction. The number of oxides having a size of 3 μm or more existing in a parallel microscopic cross-sectional area of 160 mm ² is 100 or less, of which 2 or less is 10 μm or more, and the composition ratios by composition are defined as follows: A high carbon chromium bearing steel characterized in that the total number of alumina and spinel based on the above is less than 60% of the total number of oxides.
Alumina system: (MgO) and (SiO ₂ ) are less than 3%, and (CaO) and (CaO) / ((CaO) + (Al ₂ O ₃ )) are not more than 0.08.
Spinel system: (MgO) in the range of 3% to 20% and binary system with the balance being (Al ₂ O ₃ ) contain 15% (CaO) and / or 15% (SiO ₂ ). It may have a spinel crystal structure. Production of high carbon chromium bearing steel in which the chemical composition of the steel satisfies JIS G 4805 and satisfies O: 0.0009 mass% or less, Al: 0.005 mass% or less, and S: 0.005 mass% or less. A method of using a deoxidizer containing substantially no Al for deoxidation after oxidation refining by a converter or an electric furnace and subsequent component adjustment, basicity of slag in the next ladle scouring ((CaO)) % / (SiO ₂ )%) is controlled so as to be 0.8 or more and less than 3.0, followed by a vacuum degassing treatment process of 35 minutes or more. Method.