Steel wire rod for bearing steel, manufacturing method of steel wire rod for bearing steel, heat treatment method of steel bearing, steel bearing and soaking method of bearing steel

Abstract

A heat treatment method of manufacturing high carbon bearing steel having excellent abrasion resistance and fatigue resistance, a steel wire rod for high carbon bearing steel subjected to the heat treatment, a manufacturing method of the steel wire rod, high carbon bearing steel manufactured by the heat treatment and a soaking method of a steel bloom used for manufacturing the steel wire rod. The heat treatment method of bearings includes the steps of: quenching a bearing-shaped steel part containing, by weight, 0.5% to 1.20% carbon and 1.0% to 2.0% silicon; and partitioning the quenched steel part at a temperature ranging from M.sub.s100 C. to M.sub.s for at least 10 minutes, where M.sub.s represents a temperature at which formation of martensite will start.

Claims

1. A steel bearing comprising, by weight, 0.50% to 1.20% carbon, 1.0% to 2.0% silicon, 0.20% to 1.00% manganese, 1.30% to 1.60% chromium, and up to 0.025% phosphorous, up to 0.025% sulfur, and 12 ppm or less of oxygen as impurities, wherein the steel has a microstructure comprising martensite and retained austenite, and the martensite contains no carbides, except for spheroidized carbides.

2. The bearing of claim 1, wherein the retained austenite comprises an area fraction from 5% to 15%.

3. The bearing of claim 1, having a surface and a central portion comprising martensite and retained austenite.

4. A steel wire rod for bearings for quenching and partitioning, comprising: by weight, 0.50% to 1.20% carbon, 1.0% to 2.0% silicon, 0.20% to 1.00% manganese, 1.30% to 1.60% chromium and us to 0.025% phosphorous, up to 0.025% sulfur, and 12 ppm or less of oxygen as impurities, wherein coarse carbides formed due to center segregation during casting are removed.

5. The steel wire rod for bearings of claim 4, further comprising at least one element selected from the group consisting of, by weight, up to 0.1% aluminum, up to 1.0% nickel and up to 1.0% copper.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1(a) and (b) are graphs comparing quenching-tempering (QT) treatment (A) with quenching-partitioning (QP) treatment (B);

(2) FIGS. 2(a) and (b) are micrographs showing structures of Inventive Steel 2 according to an embodiment of the present invention after qt treatment (A) and QP treatment (B);

(3) FIGS. 3 (a) and (b) are micrographs showing structures of Inventive Steel 7 according to an embodiment of the present invention after QT treatment (A) and QP treatment (B);

(4) FIG. 4 is a graph showing the results of measuring martensite start temperatures of bearing parts used in an embodiment of the present invention;

(5) FIGS. 5(a) and (b) are micrographs showing a bearing manufactured according to a conventional example (A) and a bearing manufactured according to an inventive Example (B);

(6) FIG. 6 is a micrograph showing coarse carbide formed due to the center segregation in a bloom used in an embodiment of the present invention; and

(7) FIGS. 7 (a) to (d) are a micrographs showing internal structures according to various different soaking conditions.

BEST MODE FOR CARRYING OUT THE INVENTION

(8) Hereinafter the present invention will be described in detail.

(9) As a result of intensive efforts conducted to solve the foregoing problems of the related art, the inventors of the present invention found that it is necessary not only to form martensite as an internal structure but also to ensure a retained austenite structure in order to improve the toughness of bearings, and based on this finding, have devised the present invention.

(10) However, according to the component system of a typical high-C chromium bearing, when the bearing is subjected to austenization and rapid cooling in order to form martensite, a sufficient amount of retained austenite fraction cannot be achieved in the bearing. Further, the retained austenite is also vulnerable to so-called transformation induced plasticity in which the retained austenite is transformed again into martensite due to load applied to the bearing in use. Finally, the bearing steel is composed of only tempered martensite and martensite structures.

(11) In addition, since the transformation from retained austenite into martensite is accompanied by a change in dimensions, the dimension precision of the bearing is severely damaged by the transformation. This can cause a noise problem or rapidly accelerate fatigue fracture and abrasion of the bearing.

(12) Thus, in the bearing steel, it is no less important to stabilize the retained austenite so that the retained austenite does not transform into the martensite even if stress is applied thereto than to form the retained austenite.

(13) Accordingly, the inventors have concluded the following prerequisites for realizing the purpose of the present invention: The composition of bearing steel should be controlled to be different from those of conventional bearings and a novel method of Quenching-Partitioning (QP) should be applied in order to form retained austenite at a suitable fraction as well as stabilize the retained austenite.

(14) The QP treatment indicates quenching and partitioning, and was first proposed in International Publication No. WO2004/022794. The partitioning is a processing method in which quenched steel including martensite and retained austenite is heated and maintained at a low temperature such that carbon (C) in the martensite diffuses into the retained austenite to thereby further stabilize the retained austenite at the low temperature. FIG. 1 is a view comparing the QT treatment (a) with the QP treatment (b). In FIG. 1, M.sub.s represents the temperature at which the formation of martensite will start (hereinafter, referred to as martensite start temperature), and M.sub.f represents the temperature at which the formation of martensite will finish (hereinafter, referred to as martensite finish temperature). As can be seen from the drawing, the quenching temperature is more preferably between M.sub.s and M.sub.f since it is important to maintain the retained austenite by the QP process.

(15) Accordingly, the partitioning forms the martensite, which contains substantially no carbide, and the stable retained austenite.

(16) However, the QP treatment known in the art is not directly applicable to bearing steel. Since the bearing steel contains a large amount of C, even if the bearing steel is processed at a partitioning temperature, C will precipitate inside the martensite with high probability without diffusing into the retained austenite. This causes a problem in that the original purpose of stabilizing the retained austenite cannot be realized.

(17) Therefore, it is necessary to change the composition of steel such that C can easily diffuse into the retained austenite in the partitioning to thereby stabilize the retained austenite. For this purpose, the inventors found that it is effective to set C content in steel to be in the range from 0.50 wt % to 1.20 wt % and change silicon content in steel to the range 1.0 wt % to 2.0 wt %, which is significantly higher than that contained in 100Cr6 (SAE52100=JIS-SUJ2) bearing, which has been widely used in the related art, in order to effectively diffuse C into the retained austenite in the partitioning.

(18) Like C, silicon (Si) is an element in Group 4 of the periodic table and competes for atomic sites with C. Since C becomes thermodynamically unstable at a high Si content, it diffuses from martensite into retained austenite having higher solubility. As a result, the retained austenite is further stabilized to thereby obtain the effect that the present invention intends. Hereinafter the reasons why the contents of C and Si are limited as above will be described in name detail.

(19) C: 0.50 to 1.20 Wt %

(20) Carbon (C) is not only a very important element to secure the strength of a bearing but also essential to stabilize a retained austenitic structure according to the present invention. C content is preferably 0.50 wt % or name. At a lower C content, the bearing is not suitable for mechanical parts due to low strength and low fatigue strength. In contrast, the upper limit of the C content is set 1.20 wt %. At a higher C content, the presence of unsolved coarse carbide lowers fatigue strength and decreases workability when steel is not yet quenched. A name preferable content of the bearing steel can be divided from the above range into two patterns. One of the patterns is bearing steel having a C content ranging, by weight, from 0.95% to 1.05%, which is similar to that of high-C bearing steel classified as 100Cr6 (SAE52100=JIS-SUJ2) of the related art. The other one has a C content ranging from 0.5 to 0.7 by weight percent, which is similar to that of medium-C bearing steel.

(21) Si: 1.02.0 Wt %

(22) In order to enable partitioning, which was difficult to perform in conventional bearings as described above, Silicon (Si) is preferably added in an amount of 1.0 wt % or more. However, the amount of Si is also required not to exceed an upper limit 2.0 wt %. An excessive Si content may cause decarbonization due to site competition with C and decrease workability like C when steel is not yet quenched.

(23) In the present invention, a bearing and a steel wire rod for manufacturing the bearing have a composition including, by weight, 0.5% to 1.20% C and 1.0% to 2.0% Si. Herein, the terminology including should be construed as an open limitation that includes the above-described elements but not excludes others. Those skilled in the art can include other elements in the steel composition by selecting the other elements from a variety of compositions and standardizations of conventional bearing steels and from the related art as set forth above in the background art.

(24) As an instance, the bearing or steel wire rod may preferably include at least one of the following elements. Below, it will be described of the elements that can be added to the steel composition.

(25) Mn: 0.20 to 1.00 Wt %

(26) Manganese (Mn) is an important element to secure the strength of steel by improving hardenability. Mn is preferably contained in an amount of 0.20 wt % or name but not exceeding 1.00 wt % since an excessive amount of Mn decreases workability when steel is not yet quenched. In consideration of these reasons, Mn is name preferably added in an amount from 0.25 to 0.45 by weight percent.

(27) Cr: 0.10 to 1.60 Wt %

(28) Chromium (Cr) is an element that secures the strength of steel by improving hardenability and is effective for microstructural refinement of steel. Cr is added preferably in an amount of 0.10 wt % or name, and name preferably, in an amount of 1.30 wt % or name to maximize effects, but not exceeding 1.60 wt % since its effects are saturated at an excessive amount.

(29) Al: 0.1 Wt % or Less

(30) Aluminum (Al) can be included in steel since it is an element that serves as a strong oxidizing agent in steel production, has a steel-purifying effect, and refines grains by forming a compound with nitrogen in steel. However, Al content to be added is preferably 0.1 wt % or less since Al exceeding 0.1 wt % rather decreases the steel-purifying effect and fatigue lifetime. The lower limit of the Al content is not specifically defined since Al is not necessarily added.

(31) Ni: 1.0 Wt % or Less

(32) Nickel (Ni) is an element that improves the hardenability of steel and enhances the toughness of a quenched part, and is added in an upper limit 1.0 wt %. When copper (Cu) is added, the amount of Ni added is preferably half that of copper in order to avoid thermal embrittlement. The layer limit of the Ni content is not specifically defined since nickel is not necessarily added.

(33) Cu: 1.0 Wt % or Less

(34) Copper (Cu) is added for the effect of raising the hardness of a quenched part by improving hardenability, but in an amount not exceeding 1.0 wt % since its effect is saturated when added in an amount exceeding 1.0 wt %. The lower limit of the Cu content is not specifically defined either since Cu is not necessarily added.

(35) In addition, impurities may be unavoidably included in bearings and/or in a manufacturing process thereof up to an amount that may not have an adverse effect on bearings. Those skilled in the art can manufacture a steel wire rod for bearings, in which impurity contents are controlled, and bearings from the steel wire rod by easily limiting the amount of the impurities and performing refinement according to a desirable range. Of the impurities, since phosphorous (P), sulfur (S), oxygen (Tot.O) and titanium (Ti) have relatively greater influences on the physical properties of bearings, and their contents will be specially described.

(36) P: 0.025 Wt % or Less

(37) Since phosphorous (P) is an element that decreases toughness when segregated in grain boundaries, P content can be positively limited. Regarding loads of steelmaking process etc., the P content is limited to an amount not exceeding 0.025 wt %, and preferably, not exceeding 0.02 wt %.

(38) S: 0.025 Wt % or Less

(39) While sulfur (S) acts to improve the machinability of steel, it adversely decreases toughness like P when segregated in grain boundaries and decreases fatigue lifetime by forming sulphide when bonded with Mn. Hence, it is preferable to limit the content of S. Regarding loads steelmaking process etc., the S content is limited to an amount not exceeding 0.025 wt %, and preferably, not exceeding 0.02 wt %.

(40) O: 12 Ppm or Less

(41) Oxygen (O) is an indicator of oxide inclusion and has an effect on fatigue strength. When the content of 0 in steel is high, a large amount of oxide inclusions are distributed to have an adverse effect of decreasing the fatigue strength of bearings. Preferably, O is included in an amount of 12 ppm or less based on Tot.O by an oxygen/nitrogen (0/N) analyzer.

(42) Ti: 0.01 Wt % or Less

(43) Titanium (Ti) decreases fatigue lifetime by forming coarse nitride when bonded with N. Preferably, the content of Ti is strictly limited to 0.01 wt % or less.

(44) When the bearing steel of the present invention as described above is processed into bearings, followed by quenching and partitioning heat treatment, C is properly partitioned such that retained austenite in the steel are sufficiently stabilized. This as a result can advantageously overcome a problem of the related art, in which retained austenite would otherwise transform into martensite via transformation induced plasticity when bearings are used.

(45) According to another aspect, the present invention includes procedures of austenitizing a bearing-shaped part having the above-described composition, quenching the bearing-shaped part, and then partitioning the bearing-shaped part by heat treatment. The quenching is similar to those well-known in the field of the present invention. The quenching has to be performed to quickly cool down the bearing-shaped part (water or oil cooling) to a temperature of martensite start temperature M.sub.s or under. The martensite start temperature M.sub.s can be expressed by the following Equation (1):
M.sub.s( C.)=512453C16.9Ni+15Cr9.5Mo+217(C).sup.271.5(C)(Mn)67.6(C)(Cr)Equation (1),

(46) where C, Ni, Cr, Mo, Mn and so on indicate wt % of respective elements.

(47) The following partitioning has to be determined by carefully considering compositional characteristics of the bearing steel of the present invention. In the present invention, it is necessary to promote the diffusion of C by Si through increasing Si content over that of typical bearings so that a large amount of C atoms existing in martensite can diffuse into retained austenite. Here, the partitioning temperature has to be defined since C may be distributed between a martensitic structure and a retained austenitic structure differently from the intention of the present invention so that the retained austenite is not positively stabilized if the partitioning temperature is not precisely controlled.

(48) For reliable distribution of C, it is advantageous that Si maintains its site but C diffuses as being unstabilized by Si. In other words, it is preferable to prevent Si from diffusing but to allow C to diffuse. Here, the partitioning temperature is preferably in the range from M.sub.s100 C. to M.sub.s since it is necessary to diffuse C as much as possible while keeping Si from diffusing. Meanwhile, since excessive diffusion time may be spent due to a slay diffusion rate of carbon when the temperature M.sub.s100 C. is too low according to the composition of steel, the lower limit of the partitioning temperature is preferably set 100 C.

(49) Therefore, the quenching-partitioning of the present invention is carried out by quenching the bearing-shaped part, followed by heating the part at a temperature between M.sub.s100 C. and M.sub.s, and then maintained at the same temperature for a predetermined time. Here, the partitioning time indicates a time period that is sufficient for the diffusion of C, and is not specifically limited in the present invention since those skilled in the art can easily determine a partitioning time without having to excessively repeat experiments. As an example, a time period of 10 minutes or name will be enough. Although it is not necessary to specifically limit the upper limit of the partitioning time, 30 minutes or less may be preferable in view of decrease in productivity.

(50) Accordingly, the heat treatment method of bearings according to an aspect of the present invention includes quenching a bearing part, which is machined into the shape of a bearing with an advantageous composition of the present invention, and partitioning the quenched bearing part at a temperature ranging from M.sub.s100 C. to M.sub.s for at least 10 minutes.

(51) According to another aspect of the present invention, there is provided a unique process of quenching and partitioning, which is name suitable to the composition of alloy of the present invention.

(52) As described above, the quenching is similar to a typical quenching process. However, the following partitioning is different from the disclosure of the above-described aspect of the present invention or International Publication No. WO2004/022794. Here, a name unique process is provided to ensure the hardness and toughness of bearing steel after heat treatment.

(53) In other words, another aspect of the present invention does not perform heat treatment right after the quenching but requires a procedure of maintaining a quenched bearing for a predetermined time or name. As another characteristic, the present invention has a process of quenching, maintaining and partitioning steel. Herein, the term maintaining means maintaining a specimen at a quench stop temperature without heating the specimen.

(54) According to the researches of the inventors, maintaining the quenched specimen is a very effective procedure to stabilize retained austenite. Since the specimen right after the quenching has a temperature difference between the surface and the central portion, structure inhomogeneity may occur when the specimen is heated right after the quenching. To prevent such an adverse effect, a predetermined maintaining time is necessary after the quenching. The maintaining time after the quenching is preferably one (1) minute or name. The upper limit of the maintaining time is not necessarily limited since continuing to maintain the specimen after the quenching does not cause a problem. Alternatively, the maintaining time can be set 30 minutes or less in terms of productivity since an excessive maintaining time may decrease productivity.

(55) The maintaining of the bearing after the quenching is preferably followed by partitioning. As described above, the partitioning temperature is limited to the range from M.sub.s100 C. to M.sub.s. The partitioning temperature is preferably M.sub.s100 C. or name since an intended effect of the partitioning is rarely obtainable when the partitioning temperature is too low. In the meantime, the lower limit of the partitioning temperature is name preferably 100 C. since excessive diffusion time may be spent due to a slow diffusion rate of carbon when the temperature M.sub.s100 C. is too low according to the composition of steel. In contrast, excessively high partitioning temperature is not preferable either since tempering embrittlement may occur. In addition, back-diffusion of Si is also possible although the partitioning is required to diffuse only C into austenite area. Therefore, the partitioning temperature is preferably limited to a temperature of M.sub.s or less.

(56) The partitioning can be performed preferably for at least 10 minutes in order to ensure a sufficient partitioning effect. A long partitioning time will not cause a particular problem since the partitioning is one-sided procedure of diffusing C solved in martensite toward retained austenite. Hence, it is not necessary to particularly set the upper limit of the partitioning time. Preferably, the partitioning time can be set 30 minutes or less since increasing the partitioning time may decrease productivity and the partitioning time exceeding 30 minutes does not provide a further significant effect. As such, the partitioning time is significantly shorter than that required in the tempering of the conventional QT process to thereby greatly contribute to increasing productivity.

(57) Accordingly, the heat treatment method of bearings according to an aspect of the present invention includes quenching a bearing part, which is machined into the shape of a bearing with an advantageous composition of the present invention, maintaining the quenched bearing part for at least one minute, and partitioning the maintained bearing part at a temperature ranging from M.sub.s100 C. to M.sub.s for at least 10 minutes.

(58) While the bearing manufactured by the conventional QT process has an internal structure where tempered martensite and carbide are formed, the bearing manufactured by an aspect of the present invention has an internal structure where martensite and retained austenite exist.

(59) In addition, the retained austenite in the bearing steel manufactured according to the present invention has an area fraction preferably from 5% to 15%. The area fraction is preferably 5% or more since the retained austenite is an essential structure to increase the toughness of the bearing as described above. When the fraction of the retained austenite is excessively high, the retained austenite may not be sufficiently stabilized since C diffuses in a dispersed state. As an adverse effect, part of the retained austenite may transform into martensite. This may severely damage the dimension precision such that the bearing can be damaged in use due to defective dimension precision. Accordingly, the upper limit of the fraction of the retained austenite is preferably set 15%.

(60) In addition, the bearing of the present invention may have a structure in which carbide is substantially absent in martensite unlike the bearing manufactured by the conventional QT process. Alternatively, when a wire rod manufactured as described below is subjected to drawing and spheroidization, carbide can be formed in part of martensite by spheroidizing heat treatment. The bearing of the present invention can include this type of carbide. The carbide formed by the spheroidization has a grain size of about 0.5 m or more but, preferably, not exceeding 2 m. This type of carbide can be easily discriminated from ultra-fine carbide formed by the conventional quenching and tempering. Since the carbide formed by the conventional quenching and tempering is too fine, it can be observed using an electron microscope but cannot be observed by an optical microscope. In contrast, the carbide formed by the spheroidization is relatively coarse and thus is easily identifiable. The presence of the carbide formed by the spheroidization is acceptable since it does not have a significant adverse effect on toughness but rather increases abrasion resistance.

(61) The bearing of the present invention having the above-described advantageous features is machined from a wire rod for bearing steel into the form of a bearing through drawing and spheroidization according to a typical method and is then provided to heat treatment. Therefore, the wire rod for bearings of the present invention is required to have the composition of bearing steel as described above, which is advantageous for stabilizing retained austenite by the partitioning of C in the quenching and partitioning. Particularly, the composition contains, by weight, 0.50% to 1.20% C and 1.0% to 2.0% Si. Other additional elements and impurities have been described already hereinbefore.

(62) The wire rod for bearing steel can be preferably manufactured by a process of soaking and rolling a bloom to wire rod and annealing the wire rod, wherein the annealing is carried out at a very slow rate 1 C./sec or less such that the bloom has physical properties suitable to the following drawing and spheroidization. Due to the very slow annealing, the ductility of steel is improved to thereby facilitate drawing.

(63) While the bloom provided for the manufacturing of a wire rod can be preferably soaked by internal segregation formed in casting, a soaking method for novel component systems was not proposed up to the present. Accordingly, a further aspect of the present invention provides a soaking method for the component systems, which will be described in detail as follows. Below, a description will be made of process temperature and time since they are especially important in the soaking.

(64) Process Temperature: 1190 to 1250 C.

(65) A process temperature of 1190 C. or higher is required to facilitate the diffusion of C into a component system of the present invention. A low process temperature is not preferable since coarse carbide in the central portion of a bloom will not be easily solved or diffused. In contrast, at an excessively-high process temperature, an unnecessarily-large amount of energy is consumed and decarbonization occurs in the surface of the bloom. Since the decarbonized part is not usable for a bearing due to poor hardness, hot scarfing is required to remove the decarbonized part. This however may cause problems such as increased process load and material loss. Accordingly, the upper limit of the process temperature is set 1250 C.

(66) Process Time: 1 Hour or More

(67) Process time is also a factor that has a great effect on the diffusion of segregated component. A process time of at least one hour is required to induce sufficient diffusion. More preferably, a process time of 1.5 hours or name is required. The upper limit of the process time is not specifically required since increasing the process time will not decrease soaking efficient. Preferably, the upper limit is set 6 hours in consideration of the decarbonization of material and process costs.

(68) Accordingly, the soaking method of a bloom for bearings according to an aspect of the present invention has a process temperature ranging from 1190 C. to 1250 C. and a process time of at least one hour.

(69) Hereinafter, the present invention will be described in more detail with respect to Examples. It should be understood, however, the following Examples will be provided by way of example but do not limit the scope of the present invention. Rather, the scope of the present invention shall be defined by the appended claims and equivalents thereof.

EXAMPLES

(70) Observation of Effects of Bearing Composition

(71) Bearing type parts having the composition as reported in Table 1 below were prepared in order to observe partitioning effects according to components. Any components which are not reported in the table can be included with a trace amount, which does not have a significant effect.

(72) TABLE-US-00001 TABLE 1 Samples C Si Mn P S Cr O CS.sup.1) 1 0.99 0.25 0.34 0.009 0.008 1.47 0.0010 IS.sup.2) 1 1.00 1.04 0.35 0.012 0.008 1.53 0.0009 IS 2 1.00 1.52 0.34 0.010 0.009 1.48 0.0008 IS 3 0.98 1.71 0.35 0.009 0.010 1.46 0.0008 IS 4 0.99 2.00 0.34 0.010 0.009 1.48 0.0010 CS 2 0.55 0.25 0.80 0.007 0.011 0.15 0.0012 IS 5 0.54 1.02 0.83 0.007 0.008 0.15 0.0011 IS 6 0.55 1.51 0.82 0.008 0.010 0.14 0.0009 IS 7 0.55 1.72 0.85 0.018 0.011 0.15 0.0011 IS 8 0.56 2.00 0.85 0.007 0.009 0.14 0.0011 Note) CS.sup.1)Comparative Steel, IS.sup.2)Inventive Steel

(73) Bearings were manufactured from Comparative Steel 1 and Inventive Steels 1 to 4, which have compositions of high-C steel bearings, by heating steel at 840 C. for 30 minutes, followed by rapidly cooling the steel to 70 C., which was maintained for 5 minutes, and then maintaining the steel at a temperature M.sub.s10 C. for 10 minutes, where M.sub.s is derived from Equation 1 above. Quenching and tempering (QT) were also performed in order to compare the effect of QT treatment with that of quenching-partitioning (QP) treatment on the respective high-C bearings. The quenching was performed in the same way, but the tempering was carried out by heating at a raised temperature of 180 C., which was maintained for 30 minutes.

(74) In addition, bearings were manufactured from Comparative Steel 2 and Inventive Steels 5 to 8, which have compositions of medium-C steel bearings, by heating steel at 900 C. for 30 minutes, followed by rapidly cooling the steel to 150 C., which was maintained for 10 minutes, and then maintaining the steel at 250 C. for 10 minutes. Quenching and tempering were also performed in order to compare the effect of QT treatment with that of QP treatment on the respective high-C bearings. The quenching was performed in the same way, but the tempering was carried out by heating at a raised temperature of 260 C., which was maintained for 30 minutes.

(75) The hardness and retained austenite fraction of the bearings manufactured in the above-described process were measured and the results are reported in Table 2 below:

(76) TABLE-US-00002 TABLE 2 Retained austenite Hardness fraction Samples (HRC) (%) Samples QT QP QT QP CS.sup.1) 1 60.1 60.4 0.87 2.62 IS.sup.2) 1 60.3 62.0 1.33 7.85 IS 2 60.6 62.4 1.07 8.41 IS 3 60.2 62.3 1.64 8.00 IS 4 61.5 64.7 2.83 9.27 CS 2 56.9 57.4 2.77 3.24 IS 5 57.5 61.1 3.04 7.03 IS 6 57.9 61.2 2.98 6.87 IS 7 58.1 61.7 3.17 7.20 IS 8 58.8 62.3 3.31 7.49 Note) CS.sup.1)Comparative Steel, IS.sup.2)Inventive Steel

(77) From Table 1 above, it can be understood that retained austenite fractions were under 4% in Comparative Steels 1 and 2, which have a relatively lower Si content out of the range of the present invention, even after the quenching and partitioning (QP) treatment. This retained austenite fraction is lower than that of Inventive Steels, which is typically 6.87% or more. At the retained austenite fraction under 4%, a sufficient amount of C does not diffuse into retained austenite, and thus the bearings have poor toughness.

(78) It can also be understood that each of Comparative Steels 1 and 2 has a lower hardness compared to a respective group of Inventive Steels 1 to 4 and Inventive Steels 5 to 8 when C contents are similar. This shays that a desirable level of hardness can be rarely obtained when Si content is not sufficient.

(79) In addition, it can also be appreciated that the quenching and partitioning treatment caused much higher hardness and retained austenite fraction than the quenching and tempering treatment at the same composition.

(80) FIGS. 2 and 3 compare the case in which Inventive Steels 2 and 7 were quenched and tempered (a) with the case in which Inventive Steels 2 and 7 were quenched and partitioned (b). As can be seen from the figures, the case (a) has significantly smaller retained austenite fractions than the case (b) has.

(81) Observation of Effects of Hold Time After Quenching

(82) Inventive Example

(83) A steel wire rod having the following composition of Fe0.972%C1.71%Si0.349%Mn0.019%P0.009%S0.032%Al1.46Cr0.051Ni0.009Cu0.005Ti0.0009O(oxygen) was drawn, spheroidized, and then formed into the shape of bearings.

(84) After heating at 840 C. for 30 minutes, quenching for obtaining a martensitic microstructure was performed by rapid cooling in oil having a temperature 100 C. under M.sub.s. Next, the temperature was maintained for 5 minutes and then raised to 150 C., which was maintained for 10 minutes, thereby manufacturing a bearing of the present invention.

(85) Conventional Example

(86) A conventional bearing was manufactured by quenching in the same fashion as Inventive Example, followed by tempering at 180 C. for 1 hour.

(87) Structure Comparison

(88) The internal structures of the bearings produced by Inventive and Conventional Examples are shown in FIG. 5. As can be seen from micrographs of FIG. 5, the bearing of Conventional Example (FIG. 5(a)) has tempered martensite as its main internal structure with carbide distributed in the tempered martensite. In contrast, the internal structure of Inventive Example (FIG. 5(b)) is composed of martensite and retained austenite in which substantially no carbide exists. As for retained austenite fractions, the retained austenite fraction of Conventional Example is merely 0.64% but the retained austenite fraction of Inventive Example is 8.0%, which is within the range of retained austenite fraction according to the present invention.

(89) Hardness

(90) The hardness of Conventional Example is compared with that of Inventive Example. While Conventional Example has hardness 60.2HRC, Inventive Example has hardness on the order of 62HRC. This shows that the hardness of Inventive Example is the same as or superior to that of Conventional Example.

(91) Accordingly, it is appreciated that the rolling fatigue lifetime of bearing steel can be increased by high hardness when bearings are produced according to the manufacturing method of the present invention.

(92) Observation of Effects of Soaking

(93) Specimens having a 20 mm20 mm20 mm size were extracted from the central portion of a continuous cast bloom having a 300 mm400 mm sectional area. The bloom had a tundish composition of Fe0.972%C1.71%Si0.349%Mn0.019%P0.009%S0.032%Al1.46Cr0.051Ni0.009Cu0.005Ti0.0009O. FIG. 6 is a micrograph showing coarse carbide formed inside the bloom.

(94) The extracted specimens were subjected to soaking according to a variety of temperatures and hold times as reported in Table 3 below. The cross sections of the specimens were observed to check whether or not coarse carbide was completely removed, and the results are also reported in Table 3. In the following table, the mark x indicates the residue of coarse carbide, which was not completely removed, and the mark indicates that coarse carbide was completely removed.

(95) TABLE-US-00003 TABLE 3 1190 C. 1200 C. 1210 C. 1225 C. 1240 C. 1250 C. 0.5 hr X X X X X X 1 hr X X 1.5 hrs 2 hrs 3 hrs 4 hrs 5 hrs 6 hrs

(96) From the table above, it can be understood that carbide cannot be completely removed irrespective of temperature when the soaking was performed for 0.5 hour. When soaking was performed for 1 hour, a complete soaking effect was not obtained due to the residue of a certain amount of coarse carbide at heat treatment temperatures 1190 C. and 1200 C. In other temperatures, sufficient treatment effects were obtained since a slight amount of fine carbide remained and substantially no coarse carbide existed.

(97) At treatment times of 1.5 hours or more, it was observed that coarse carbide was completely removed at a temperature 1190 C., which is defined by the present invention.

(98) To support this, FIG. 7 shows micrographs of the specimens according to treatment temperature and time of some of the factors reported in Table 1. FIG. 7(a) represents the result of treatment at 1200 C. for 0.5 hour, and 7(b) represents the result of treatment at 1225 C. for 0.5 hour. FIGS. 7(a) and 7(b) shay that coarse carbide was rarely removed. In contrast, FIG. 7(c) shows the result of treatment at 1200 C. for 1.5 hours in which coarse carbide was almost removed and part of fine carbide remained. The fine carbide rarely has an adverse effect on the fatigue lifetime of bearings compared coarse carbide. FIG. 7(d) shows the result of treatment at 1225 C. for 1 hour in which coarse carbide was also excellently removed.

(99) Accordingly, advantageous effects of the present invention can be appreciated.