Abstract:
Disclosed is a method for producing a mechanical part superior in rolling fatigue life. The mechanical part includes a rolling portion for a rolling element to roll along an inner diameter surface of an annular workpiece material subjected to cold forging. The method includes performing cold forging by applying a hydrostatic stress to the inner diameter surface of the annular workpiece material on which the rolling portion is to be formed, thereby forming the rolling portion on the inner diameter surface of the annular workpiece material for the rolling element to roll therealong and increasing an inner diameter of the annular workpiece material other than the rolling portion, so as to provide an annular mechanical part including the rolling portion superior in rolling fatigue life.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Japanese Patent Application No. 2009-194962 filed on Aug. 26, 2009 and Japanese Patent Application No. 2010-185927 filed on Aug. 23, 2010, the entire disclosures of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to production of a mechanical part made of a steel material, such as a bearing, a gear, a hub unit, a variable speed transmission, a constant velocity joint or a piston pin, particularly to production of a mechanical part comprising an annular body for which a good rolling fatigue life is required. 
       BACKGROUND ART 
       [0003]    In recent years, with increasingly high performance in various mechanical apparatuses, usage environments of mechanical parts or apparatuses for which a rolling fatigue life is required become very severe, causing a strong demand for improvements in operating life and reliability of these mechanical parts or apparatuses. In response to such a demand, as a measure in terms of steel materials, there has been conducted proper adjustment of steel ingredients or reduction of impurity elements contained along with the steel ingredients. 
         [0004]    Among the impurity elements contained along with the steel ingredients constituting these mechanical parts or apparatuses, Al 2 O 3 , MnS, TiN and other non-metallic inclusions consisting of these impurity elements may originate a failure in steel parts of the mechanical parts or apparatuses. For this reason, it is known that these non-metallic inclusions are particularly detrimental. It is further known that a larger diameter of the non-metallic inclusion leads to a shorter rolling fatigue life of steel parts. Thus, there has been proposed various high cleanliness steel having a reduced amount of non-metallic inclusions, with the purification level of the steel being heightened and with an extremely reduced amount of large oxide-based non-metallic inclusions having diameters of 20 μm or more (for example, see Patent Literature 1 and Patent Literature 2). 
         [0005]    In the meantime, even if a steel material made of such high cleanliness steel is used for a mechanical part or apparatus, it has not yet been sufficiently achieved to prevent the mechanical part or apparatus from leading to failure in a short life span. For this reason, developments are being actively conducted to reduce the amount of non-metallic inclusions in the steel material and further to reduce the diameters of the non-metallic inclusions. 
         [0006]    On the other hand, technological developments are also being actively conducted to provide mechanical parts excellent in rolling fatigue life even without reducing the amount of the non-metallic inclusions in the steel material and without reducing the diameters thereof. For example, there has been proposed: (1) a technique in which, at the time of producing a part by rolling motion, a fiber flow on a rolling portion is controlled to obtain an excellent rolling fatigue life (for example, see Patent Literature 3) and (2) a technique in which a compression stress is applied in advance to a rolling portion to obtain an excellent rolling fatigue life (for example, see Patent Literature 4). In addition, the applicant has proposed (3) a technique of providing a steel material with an improved interface state between non-metallic inclusions contained in the steel material and the matrix steel to achieve an excellent rolling fatigue life. These techniques are published in Non-Patent Literature 1 and Non-Patent Literature 2. 
         [0007]    In Non-Patent Literature 1 and Non-Patent Literature 2, the process leading to a rolling fatigue failure, namely flaking, is explained as follows. That is, in the process that cracks originated from the non-metallic inclusion lead to the generation of flaking through their growth, there is involved a crack initiation stage (hereinafter, “Mode I-type initial crack”) in which the crack is displaced due to a stress concentration effect onto the periphery of the non-metallic inclusion. It is known that this then leads to failure through propagation of the crack by shear stress. This means that if a Mode I-type initial crack is not generated, subsequent crack propagation or failure does not occur. In addition, the Mode I-type initial crack occurs on the premise that there is generated a physical cavity, in an interface between the non-metallic inclusion and the matrix. It is also verified that unless the physical cavity is generated, the Mode I-type crack is not generated. 
         [0008]    Meanwhile,  FIG. 5  is a conceptual diagram showing an image of the periphery of the non-metallic inclusion in a test piece which was cut out from a hot rolled steel material and then subjected to ion milling, when observed by a scanning electron microscope (FE-SEM) to confirm presence/absence of cavities therein. In 
         [0009]      FIG. 5 , code  5  indicates a non-metallic inclusion of Al 2 O 3 , while code  4  indicates cavities. Particularly in machine structural steel, deoxidation by Al is normally performed. An Al 2 O 3 -based non-metallic inclusion  5  formed at this time is confirmed to tend to easily generate cavities  4  particularly in an interface with the matrix due to the difference in deformability from the matrix or due to the shape. Therefore, in order to improve the rolling fatigue life of the mechanical part  7 , it is effective to close the cavity  4  existing in the interface between the non-metallic inclusion  5  and the matrix or to reduce the volume of the cavity  4 . 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP2006-63402A 
         Patent Literature 2: JP06-192790A 
         Patent Literature 3: JP4-357324A 
         Patent Literature 4: JP2006-77854A 
       
     
       Non-Patent Literature 
       [0000]    
       
         Non-Patent Literature 1: Tetsu-to-Hagane, 94 (2008), p.13 
         Non-Patent Literature 2: Kazuhiko Hiraoka, Heisei 20-Nendo Academic Dissertation of University of Hyogo (January in 2008) 
       
     
       SUMMARY OF INVENTION 
       [0016]    The present invention is directed to a technology relating to (3) described in the above paragraph [0006] and is to provide a method for producing a mechanical part having a rolling portion superior in rolling fatigue life on the inner diameter surface of an annular workpiece material, by conducting plastic processing to improve the state of the interface between non-metallic inclusions and the matrix steel material contained in the steel material constituting the annular workpiece material, which is to be subjected to cold forging, as compared to the conventional method for producing the conventional steel material aiming at reducing the non-metallic inclusions and reducing the diameters of the non-metallic inclusions. 
         [0017]    According to the present invention, there is provided a method for producing a mechanical part superior in rolling fatigue life, wherein the mechanical part comprises a rolling portion for a rolling element to roll along an inner diameter surface of an annular workpiece material subjected to cold forging, wherein the method comprises the step of performing cold forging by applying a hydrostatic stress to the inner diameter surface of the annular workpiece material on which the rolling portion is to be formed, thereby forming the rolling portion on the inner diameter surface of the annular workpiece material for the rolling element to roll therealong and increasing an inner diameter of the annular workpiece material other than the rolling portion, so as to provide an annular mechanical part comprising the rolling portion superior in rolling fatigue life. The application of this hydrostatic pressure directs the cavities existing in the interface between non-metallic inclusions in the steel and the steel matrix to close, enabling production of a mechanical part superior in rolling fatigue life. The annular workpiece material to be subjected to cold forging is preferably a steel pipe or a hot forged ring. 
         [0018]    According to a preferred aspect of the present invention, there is provided the method in which the hydrostatic stress is at least 1000 MPa. Applying a hydrostatic stress of at least 1000 MPa during cold forging to form a rolling surface makes it possible to close the cavities existing in the interface between the non-metallic inclusions contained in the steel and the steel matrix and thus to produce a mechanical part superior in rolling fatigue life. 
         [0019]    According to the production method of the present invention, when a hydrostatic stress (preferably at least 1000 MPa) is applied to the annular workpiece material at cold forging, the cavities generated in the interface between the non-metallic inclusions and the steel matrix can be closed or reduced even without reducing the amount of the non-metallic inclusions and without reducing the diameters thereof at the time of producing the steel material. As a result, it is possible to avoid flaking which occurs due to rolling contact fatigue originated from non-metallic inclusions, enabling production of a mechanical part provided with a rolling portion having a significantly improved rolling fatigue life. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1A  is a schematic view explaining a cold forging process (before compression processing) for an annular matrix according to the present invention. 
           [0021]      FIG. 1B  is a schematic view explaining a cold forging process (after compression processing) for the annular matrix according to the present invention. 
           [0022]      FIG. 2  is a longitudinal cross-sectional view of a rolling bearing produced in accordance with the method of the present invention. 
           [0023]      FIG. 3  is a diagram showing CAE analysis of the rolling bearing produced in accordance with the method of the present invention 
           [0024]      FIG. 4A  is a conceptual diagram showing a non-metallic inclusion and cavities in the periphery thereof before cold forging. 
           [0025]      FIG. 4B  is a conceptual diagram showing a non-metallic inclusion and cavities in the periphery thereof after cold forging. 
           [0026]      FIG. 5  is a conceptual diagram showing a non-metallic inclusion of a hot rolled steel material and cavities in the periphery thereof. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    The steel material required for the production method of the present invention may be machine structural steel and bearing steel. 
         [0028]    These machine structural steels are generally produced as a steel material through 1) oxidation refining of molten steel in an arc melting furnace or a converter furnace, 2) reduction refining in a ladle refining furnace (LF), 3) rotary-flow vacuum degassing treatment by a rotary-flow vacuum degasser (RH treatment), 4) casting of steel ingot by continuous casting or ingot casting and 5) plastic working of steel ingot by hot rolling or hot forging and cold rolling or by cold rolling and cold forging. 
         [0029]    The annular matrix  2  used in the production method of the present invention can be produced as follows. First of all, the steel material produced as described above (for example, steel materials defined in JIS G 4805 (2008), JIS G 4051 (2005), JIS G 4104 or JIS G 4105) is subjected to the process of the aforementioned plastic working to produce a steel material. This steel material is subjected to asset milling, extrusion processing or hot processing such as hot forging to be processed into a steel pipe or a hot forged ring, which is then cut to a predetermined length. Further, the outer diameter surface and the inner diameter surface of the cut steel pipe or hot forged ring are subjected to cutting processing, and thus provide a steel pipe or hot forged ring provided with predetermined dimensions as the annular matrix  2 . 
         [0030]    The process according to the present invention will be explained with reference to  FIG. 1 . The annular matrix  2  in a predetermined configuration is subjected to appropriate lubrication processing to have a temperature around room temperature. The annular matrix  2  is set within an annular retraining frame  1  in a press apparatus as shown in  FIG. 1A . Dies  3  are arranged at upper and lower positions in the restraining frame  1  and are respectively fixed to moving parts (not shown) at upper and lower positions in the press apparatus. Following the start of a processing motion of the press apparatus, an upper punch  3   a  of the fixed die  3  and an annular upper punch  3   b  arranged around the upper punch  3   a  start a descending motion in the arrow direction. In the annular matrix  2  set in a predetermined position in the dies  3 , the descending upper punch  3   a  and annular upper punch  3   b  applies plastic working to the inner diameter  2   a  and the upper end face  2   b  of the annular matrix  2 . In addition, following the descending of the upper punch  3   a  and the annular upper punch  3   b,  the annular matrix  2  is pushed downward, while at the same time a lower punch  3   c  and an annular lower punch  3   d  of the die  3  applies plastic working to the inner diameter  2   a  and the lower end face  2   c  of the annular matrix  2 . That is, the upper end face  2   b  of the annular matrix  2  is pushed downward following the descending of the upper punch  3   a  and the annular upper punch  3   b.  As a result, the lower end face  2   c  of the annular matrix  2  is relatively pushed up by the lower punch  3   c  and the annular lower punch  3   d.  In the end stage of the processing, the annular matrix  2  is subjected to compression processing to receive a hydrostatic stress by cold forging from the upper punch  3   a  and the annular upper punch  3   b  as well as the lower punch  3   c  and the annular lower punch  3   d,  thereby closing the cavities  4  existing between the steel matrix of the annular matrix  2  and the non-metallic inclusions  5 . 
         [0031]    As shown in  FIG. 1B , subjecting the annular matrix  2  to the above compression processing applies a hydrostatic stress to the vicinity of the rolling portion  6  in the mechanical part  7  to be produced, thereby providing an effect of closing the cavities  4  existing between the steel matrix of the annular matrix  2  and the non-metallic inclusions  5 . In this case, in order to sufficiently attain the effect of closing the cavities  4 , it is preferable that a hydrostatic stress of at least 1000 MPa be applied to the vicinity of the rolling portion  6  at cold forging. The application of such hydrostatic stress directs the cavities  4  existing between the non-metallic inclusions  5  and the matrix steel of the annular matrix  2  to change so as to close the cavities  4  or to reduce the volume of the cavities  4 . This change enables avoidance of flaking which occurs due to rolling contact fatigue originated from non-metallic inclusions  5 . As a result, there can be obtained the mechanical part  7  having the rolling portion  6  having a superior rolling fatigue life. 
       EXAMPLE 1 
       [0032]    The present invention will be explained with reference to examples in view of implementing conditions and obtained results. The compositions of test pieces used as steel materials for the annular matrix  2  are shown in Table 1. 
         [0000]    
       
         
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Ingredients of Test pieces 
               
             
          
           
               
                 (unit: % by mass) 
               
             
          
           
               
                 Steel Type 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cu 
                 Ni 
                 Mo 
                 Cr 
                 Al 
               
               
                   
               
             
          
           
               
                 SUJ2 
                 1.04 
                 0.22 
                 0.32 
                 0.008 
                 0.007 
                 — 
                 — 
                 0.03 
                 1.44 
                 0.011 
               
               
                 SUJ3 
                 1.00 
                 0.55 
                 1.01 
                 0.007 
                 0.008 
                 0.03 
                 0.05 
                 0.01 
                 0.99 
                 — 
               
               
                 S45C 
                 0.44 
                 0.20 
                 0.70 
                 0.01 
                 0.01 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 S53C 
                 0.52 
                 0.20 
                 0.65 
                 0.01 
                 0.01 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
         [0033]    This example was implemented on test pieces of steel types shown in Table 1. First of all, molten steel was subjected to an oxidation refining in an arc melting furnace, a reduction refining in a ladle refining furnace (LF), and a degassing processing in a rotary-flow vacuum degasser (RH) for reducing the oxygen content in the molten steel, which was then subjected to continuous casting to produce a steel ingot. The steel ingot was subjected to conventional hot rolling to provide a steel material, which was then processed to form a steel pipe by assel mill. The steel pipe was then subjected to conventional spheroidized annealing to prepare a steel pipe. 
         [0034]    The above-obtained steel pipe made of the test piece shown in Table 1 and having an outer diameter φ of 80 mm and a thickness of 8.7 mm was sawn to a steel pipe having a width of 27.2 mm in the longitudinal direction of the steel pipe, of which the outer diameter and the inner diameter were subjected to cutting processing to provide a steel pipe having an outer diameter φ of 78.5 mm and a thickness of 7.0 mm. This steel pipe was then subjected to conventional lubrication processing to provide an annual matrix  2  for cold forging. The annular matrix  2  was subjected to the following cold forging, as shown in  FIG. 2 , by using dies  3  designed in such a way that a cold forged product having a width of 28.1 mm, an outer diameter φ of 79.0 mm could be obtained in which the central portion in the inner diameter had a projection  2   d  having a width of 7.5 mm and an inner diameter φ of 61.8 mm and in which the rolling portion  6  had a largest inner diameter  2   a  of 68.2 mm. The cold forging was performed in such a way that the annular matrix  2  and the dies  3  both have temperatures around room temperature and apply a processing load of 4000 to 4200 kN and a processing surface pressure of 1800 to 1900 MPa in accordance with the processing method using the dies  3  shown in  FIG. 1 . 
         [0035]    It is considered that the cold forging applies a hydrostatic stress of up to approximately 1500 MPa to the vicinity of the rolling portion  6 , as predicted from the CAE analysis diagram shown in  FIG. 3 . 
         [0036]      FIG. 4A  and  FIG. 4B  schematically show the change of the cavities  4  existing between the non-metallic inclusion  5  and the steel as the annular matrix  2  from before to after the cold forging.  FIG. 4A  shows a configuration of the non-metallic inclusion  5  in the annular matrix  2  before the cold forging, with the cavities  4  being formed adjacent to the non-metallic inclusion  5 . As shown in  FIG. 4B , however, it was confirmed that the cavity  4  existing between the non-metallic inclusion  5  and the steel as the annular matrix  2  was closed after the cold forging. 
         [0037]    Further, in order to evaluate the rolling fatigue life of the mechanical part  7  which is the effect of the present invention, molding loads and molding methods at cold forging were controlled to obtain various test pieces under four steel type conditions and five processing conditions as shown in Table 2. 
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Steel type conditions 
               
             
          
           
               
                 Steel type 
                   
               
               
                 condition 
                 Steel type 
               
               
                   
               
               
                 1 
                 SUJ2 
               
               
                 2 
                 SUJ3 
               
               
                 3 
                 S45C 
               
               
                 4 
                 S53C 
               
               
                   
               
             
          
           
               
                 Processing conditions 
               
             
          
           
               
                   
                 Processing 
                   
                   
                 Maximum 
               
               
                   
                 initiation 
                   
                 Molding 
                 hydrostatic 
               
               
                 Processing 
                 temperature 
                 Processing 
                 load 
                 stress 
               
               
                 condition 
                 (° C.) 
                 method 
                 (kN) 
                 (MPa) 
               
               
                   
               
               
                 A 
                 20 
                 Compression 
                 4000-4200 
                 1500 
               
               
                   
                   
                 processing 
               
               
                 B 
                 20 
                 Compression 
                 2700-3000 
                 1000 
               
               
                   
                   
                 processing 
               
               
                 C 
                 20 
                 Compression 
                 3200-3500 
                 1250 
               
               
                   
                   
                 processing 
               
               
                 D 
                 20 
                 Tension 
                 2700-3000 
                 −1000 
               
               
                   
                   
                 processing 
                   
                 (tension) 
               
               
                 E 
                 1000 
                 Compression 
                 350-500 
                 800 
               
               
                   
                   
                 processing 
               
               
                   
               
             
          
         
       
     
         [0038]    These obtained test pieces were subjected to turning processing to form a bearing washer which is a member of a thrust rolling bearing, followed by quenching and tempering treatment. There were thus obtained a hardness of HRB 94 or more for S45C, a hardness of HRC 20 or more for S53C, and a hardness of HRC 58 or more for SUJ2 and SUJ3. The test pieces were further subjected to grinding to provide a thrust rolling bearing, followed by evaluation of the rolling fatigue life thereof. For the rolling body, a commercially available ball for the thrust rolling bearing was used. 
         [0039]    The evaluation results of the rolling fatigue life were shown in Table 3 in accordance with three-step criteria including A: excellent, B: good, and C: poor. For the evaluation, the number before hyphen “-” in the conditions in Table 3 refers to steel type condition in Table 2 while the alphabetical letter after hyphen “-” refers to processing condition in Table 2. Since the hardness is different from each other among steel types, the same evaluation cannot be made thereamong. Therefore, the evaluation of the rolling fatigue life was made based upon comparison among the same steel types. It was confirmed that as the maximum compression stress at cold forging increases toward 1500 MPa, the rolling fatigue life improves so as to be evaluated as “A.” On the other hand, in the case where the tension stress was applied to the vicinity of the rolling portion  6  by cold forging, the evaluation was rendered “C” which means that the rolling fatigue life was not improved, as shown in conditions “1-D,” “2-D,” “3-D,” and “4-D” in Table 3 under which the processing was conducted in accordance with processing condition D of Table 2. On the other hand, although not shown in Table 3, there is also confirmed an effect that the rolling fatigue life improves even if production was conducted with hot forging. However, the cold forging according to the present invention is more advantageous in that the steel material temperature is not raised. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Evaluation of rolling 
               
               
                   
                 Conditions 
                 fatigue life 
               
               
                   
                   
               
             
             
               
                   
                 1 - A 
                 A 
               
               
                   
                 1 - B 
                 B 
               
               
                   
                 1 - C 
                 B 
               
               
                   
                 1 - D 
                 C 
               
               
                   
                 1 - E 
                 B 
               
               
                   
                 2 - A 
                 A 
               
               
                   
                 2 - B 
                 B 
               
               
                   
                 2 - C 
                 B 
               
               
                   
                 2 - D 
                 C 
               
               
                   
                 2 - E 
                 B 
               
               
                   
                 3 - A 
                 A 
               
               
                   
                 3 - B 
                 B 
               
               
                   
                 3 - C 
                 B 
               
               
                   
                 3 - D 
                 C 
               
               
                   
                 3 - E 
                 B 
               
               
                   
                 4 - A 
                 A 
               
               
                   
                 4 - B 
                 B 
               
               
                   
                 4 - C 
                 B 
               
               
                   
                 4 - D 
                 C 
               
               
                   
                 4 - E 
                 B 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    From the evaluation results of the rolling fatigue life test implemented on the test pieces in accordance with the four steel type conditions and the five processing conditions as described above, it was found that by applying a predetermined compression stress (preferably at least 1000 MPa) to the vicinity of the rolling surface at cold forging, the cavities  4  existing between the non-metallic inclusions  5  and the steel as the annular matrix  2  were closed or reduced to achieve an improvement in rolling fatigue life. It goes without saying that similarly to the above, an improvement in rolling fatigue life can also be achieved in an outer race ring material of a double row raceway. 
       EXAMPLE 2 
       [0041]    Test pieces are produced in the same way as in Example 1, except that hot forged rings are produced instead of the steel pipes. The method for producing the hot forged ring is as follows. First of all, molten steel was subjected to an oxidation refining in an arc melting furnace, a reduction refining in a ladle refining furnace (LF), and a degassing processing in a rotary-flow vacuum degasser (RH) for reducing the oxygen content in the molten steel, which was then subjected to continuous casting to produce a steel ingot. The steel ingot was subjected to conventional hot rolling to provide a steel material, which was then processed to form a billet by shear cutting. The billet was then subjected to hot forging to provide a hot forged ring having an outer diameter φ of 80 mm, a thickness of 8.7 mm and a width of 27.2 mm. The hot forged ring was subjected to conventional spheroidized annealing to prepare a hot forged ring. The outer diameter and the inner diameter of the hot forged ring made of the above-obtained test piece shown in Table 1 were subjected to cutting processing to provide a hot forged ring having an outer diameter φ of 78.5 mm and a thickness of 7.0 mm. The hot forged ring was then subjected to conventional lubrication processing to form an annual matrix  2  for cold forging. Cold forging and its subsequent processes using the annular matrix  2  are the same as in Example 1. In this case, effects similar to those attained in Example 1 can also be attained.