JP4385921B2 - Bearing steel with excellent grindability - Google Patents

Description

  The present invention relates to a bearing steel that has fatigue characteristics and forgeability and is suitable for machine parts such as automobile parts and electric parts, and particularly relates to a steel that is excellent in workability and surface properties after polishing in a final polishing process for rolling elements.

  The steel balls of the bearing parts are molded from a bearing material by cold forging, deburred, quenched, and tempered, and then polished into a product having a predetermined accuracy in a polishing process. The polishing step is composed of a plurality of steps from rough polishing to final finish polishing called lapping.

  The final polishing process is the process that has the greatest effect on the accuracy and characteristics of parts. The steel balls are sandwiched between two polishing wheel plates, rolled, ground, and processed to the desired roundness, waviness and surface roughness. However, since it takes a long time, it is a rate-limiting process from the viewpoint of productivity.

Therefore, various techniques have been proposed for improving the polishing properties. Patent Document 1 relates to a stainless steel for bearings, which suppresses the generation of Al ₂ O ₃ and SiO ₂ by regulating the composition of the steel material, reduces the hardness difference in the steel structure, and enables high-precision machining. Is described.

  Patent Document 2 describes a wire rod having excellent polishing characteristics in which the size and number of sulfide inclusions are defined in steel in which the component composition is defined. It is characterized in that machinability is mainly improved by MnS without deteriorating cold forgeability, and polishing in a short time is possible with a certain size and number of MnS.

Patent Document 3 relates to bearing steel excellent in polishability, and defines the size of carbonitrides mainly composed of Ti by the length in the rolling direction, and specifies the size and number. It is characterized in that the grinding amount during finish polishing is suppressed with a large Ti-based carbonitride of 25 μm or more, the product accuracy is improved, the consumption of the polishing wheel is also suppressed, and the polishing efficiency is improved.
JP-A-9-137257 JP-A-10-121198 JP 2001-279393 A

  The technique described in Patent Document 1 described above mainly aims to improve the machining accuracy of the cutting process, and the techniques described in Patent Documents 2 and 3 mainly consider improvement in the productivity of polishing. The purpose is to reduce the time required for optimum polishing.

  However, since the polishing process is the final process of bearing production, it is required to finish the surface texture of the product to a mirror surface with high accuracy.

  That is, the abrasiveness cannot be evaluated only by the processing accuracy at the time of cutting and the time required for polishing, and evaluation from the viewpoint of surface roughness and gloss is also important. In order to obtain a glossy surface property, it is necessary to perform polishing using a polishing grindstone that is clogged and has a small polishing allowance. Therefore, it is generally difficult to achieve both polishing efficiency and surface gloss.

  Therefore, the present invention is based on the premise of maintaining excellent fatigue characteristics and forgeability, which are basic characteristics of bearing steel, and has excellent workability and efficiency in the finish polishing process, and an excellent surface when used as a product. An object is to provide a bearing steel capable of obtaining gloss.

  In order to achieve the above-mentioned object, the present inventors diligently studied the influence of inclusions in steel on the abrasiveness. As a result, it has been found that the size and number of TiN greatly affect the polishing properties, that is, the efficiency of the polishing operation and the surface roughness after polishing.

The present invention has been made by further study based on the obtained knowledge, that is, the present invention is 1% by mass, C: 0.6-1.5%, Mn: 0.2-1. 5%, Si: 0.05-1.2%, Cr: 0.5-2.5%, Ti: 0.002-0.020%, N: 0.008% or less, Al: 0.01- The composition is 0.03%, Ti / N ≦ 3.4, the balance is Fe and inevitable impurities, the maximum diameter of TiN in the steel is 10 μm or less, and the number of TiN is 0.25 per 1 mm ^2. Bearing steel excellent in abradability characterized by the above.

  21. Excellent polishing properties characterized by further containing one or more of Cu: 0.2% or less, Ni: 0.2% or less, Mo: 0.1% or less in the component composition described in 21 Bearing steel.

  According to the present invention, it is possible to obtain a bearing steel that has excellent fatigue characteristics and forgeability, has high workability in the finish polishing step to be a rolling element, enables high-efficiency polishing, and has excellent surface roughness after polishing. It is extremely useful.

The reason for limitation of the present invention will be described below. In addition, all content (%) of each element in a component composition means (mass%).
[Ingredient composition]
C
C is necessary for securing the strength required for bearing steel, and if it is less than 0.6%, the predetermined strength cannot be secured. On the other hand, if it exceeds 1.5%, softening is difficult and the cold forgeability is remarkably lowered, and defects such as cracks are likely to occur in the heat treatment after forging, so 0.6 to 1.5%.

Mn
Mn is an element necessary for deoxidation, and is required for improving fatigue characteristics by solid solution strengthening, so is 0.2% or more. On the other hand, if it exceeds 1.5%, the cold forgeability is remarkably deteriorated, so the content is made 0.2 to 1.5%.

Si
Si is an element necessary for deoxidation, and if it is less than 0.05%, a desired effect cannot be obtained. On the other hand, if it exceeds 1.2%, the mechanical properties such as fatigue life and particularly the cold forgeability are significantly deteriorated.

Cr
Cr has the effect of remarkably promoting the formation of cementite, and having the effect of reducing the pearlite lamellar spacing and pearlite grains, and improving fatigue characteristics as an appropriate carbide spheroidized structure, so 0.5% or more. On the other hand, even if added in excess of 2.5%, the effect is saturated, but the fatigue strength and ductility are adversely affected.

Ti
Ti combines with N to form TiN inclusions, and improves the abrasiveness when properly present in the steel. If Ti is less than 0.002%, the effective TiN is small and the polishing property is not improved. On the other hand, if it exceeds 0.020%, TiN becomes excessive, the polishing property is lowered, the progress of polishing is suppressed, and the fatigue characteristics are also markedly lowered, so 0.002 to 0.020%.

N
N combines with Ti and exists in the steel as TiN inclusions. If N exceeds 0.008%, TiN becomes coarse and does not contribute to polishing properties, and fatigue strength is reduced. Further, excessive N is dissolved to the solid solution limit amount in the steel and lowers the cold forgeability, so it is made 0.008% or less.

Al
Al is an element necessary for deoxidation like Si, and is 0.01% or more. On the other hand, if it exceeds 0.03%, the mechanical properties such as fatigue life and particularly the cold forgeability are significantly deteriorated, so the content is made 0.01 to 0.03%.

Ti / N
When Ti / N is less than or equal to the stoichiometric ratio (= 3.4), TiN becomes fine inclusions of 10 μm or less and is dispersed in the steel, improving the polishing properties and fatigue strength. On the other hand, if the stoichiometric ratio (= 3.4) is exceeded, TiN exceeding 10 μm will be present, the effect of improving the polishability will be saturated, and the fatigue strength will also be reduced, so that it will be 3.4 or less. .

  In the present invention, P, S, and O are treated as inevitable impurities. P and S segregate at the grain boundaries of the steel and embrittle the steel. Therefore, the smaller the content, the better. P ≦ 0.03% and S ≦ 0.02% are preferable.

O generates Al, Si and oxide inclusions such as Al ₂ O ₃ and SiO ₂ in steel and greatly reduces the rolling fatigue strength. Therefore, the smaller the content, the less the content, and 20 ppm or less. Is preferred.

  The above is the basic component composition of the present invention. In the case of further improving the characteristics as a bearing steel, one or more of Cu, Ni, and Mo can be added.

Cu
Since Cu enhances the hardenability of steel, it improves the strength of the bearing after processing. When adding over 0.2%, bainite and martensite are generated even in the rolled state and forgeability is lowered, so the content is made 0.2% or less.

Ni
Ni enhances the hardenability of the steel, so it improves the strength of the bearing after processing. When adding over 0.2%, bainite and martensite are generated even in the rolled state, and the forgeability deteriorates, so the content is made 0.2% or less.

Mo
Mo increases the hardenability of the steel, thus improving the strength of the bearing after processing. When adding over 0.1%, bainite and martensite are formed even in the rolled state and forgeability is lowered, so the content is made 0.1% or less.

[TiN]
In this invention, the magnitude | size and distribution state of TiN in steel are prescribed | regulated. TiN having a particle size of more than 10 μm causes uneven polishing in the final polishing step and reduces the surface roughness after polishing. Moreover, it is easy to become a starting point of fatigue fracture, and a coarse thing extremely reduces the fatigue life of steel. Therefore, the maximum diameter of TiN is set to 10 μm or less.

In the present invention, the maximum diameter of TiN can be observed using SEM, and the particle diameter means the longest diameter of TiN grains. In obtaining the maximum diameter of TiN, the observation area is set to 320 mm ² .

Further, when the number of TiN is less than 0.25 pieces / mm ^2, the surface roughness becomes excessively polished state in the polishing step is 0.25 pieces / mm ² or more so lowered.

  The bearing steel according to the present invention is manufactured by the following steps. After refining the molten steel of a predetermined component, it is cast into a slab, and after the annealing, the slab is rolled. When casting into a slab, the degree of superheated molten steel is set to 30 ° C. or less and the thickness of the slab is set to 350 mm or less so that coarse TiN is not generated.

  When the molten steel superheat degree exceeds 30 ° C., when it is cast, a segregation band is generated at the center of the slab, and coarse Ti carbonitride is generated. On the other hand, if the thickness of the slab exceeds 350 mm, the cooling rate necessary for preventing the formation of coarse TiN cannot be obtained.

  The steel slab is rolled to a desired size and then reheated, followed by steel bar rolling, spheroidizing and wire drawing. Next, forging, heat treatment, and polishing to obtain a product.

  The effect of this invention is shown with an Example. Steel having various composition shown in Table 1 is made into a steel ingot in a vacuum melting furnace, hot rolled into a steel bar, held at 790 ° C. for 5 hours, and then annealed and gradually cooled, and then subjected to Φ15. An 8 mm wire was used. About the obtained wire, abrasiveness, fatigue characteristics, and cold forgeability were evaluated.

  Steel No. 1 in Table 1 Nos. 1 to 37 were cast into a 30 kg steel ingot in a vacuum melting furnace, cast at a superheat degree of 25 ° C., and the thickness of the slab was 180 mm. No. 38 was cast at a superheat of 50 ° C., and the slab thickness was 180 mm. Steel No. No. 39 was a 150 kg steel ingot and a 360 mm square slab.

  The test piece used for the evaluation of the polishing property was obtained by cutting a cylindrical body of Φ10 mm × 10 mm from the obtained wire, holding it at 950 ° C. for 30 minutes, quenching with oil, and tempering at 180 ° C. for 2 hours. Thereafter, the test piece was embedded in a bakelite resin so that the cylindrical end surface was a polished surface, and rough polishing was performed.

  For evaluation of polishing performance, a Vickers indentation was applied to the polished surface after rough polishing with a load of 10 kg, and then the surface was polished with an abrasive polishing agent and water for 10 hours using a vibration polishing machine, and the surface roughness (Ry (μm)) was measured. In addition, the polishing amount (μm) was obtained from the change in the size of the indentation. Furthermore, the surface roughness was measured at regular intervals, and the time until Ry became 0.02 μm or less was determined.

  The maximum height Ry was measured according to JISB0601 with an evaluation length of 0.4 mm.

Observation of TiN in the steel was performed using a SEM for a test piece (Φ10 mm × 10 mm) that was kept in an oil-quenched state at 950 ° C. for 30 minutes. The observation range was 320 mm ² , the maximum diameter of TiN was obtained, and the number of TiN was measured and converted to the number per 1 mm ² .

  The fatigue test was performed using a radial rolling fatigue tester with a Hertz maximum contact stress of 5.8 GPa, lubricating oil: # 68 turbine oil, and the test results were arranged with probability paper according to the Weibull distribution to determine the B10 life. The rolling fatigue test piece was oil quenched (oil temperature 60 ° C.) after being held at 850 ° C. for 20 minutes, sampled from a wire that had been tempered at 180 ° C. for 2 hours, and was subjected to cutting and lapping finishing.

  For cold forgeability, a Φ15 × 22.5 mm cylindrical body was sampled from the drawn wire so that the axial direction was the rolling direction, and used as a test piece. In the cold forging test, compression with various rolling reductions was performed at n = 10, and evaluation was performed based on the presence or absence of cracks. FIG. 1A shows a test method, and FIG.

  Evaluation of cold forgeability calculated | required the crack generation rate about each compression rate, and made the compression rate which a crack generate | occur | produces in 50% (n = 5) of a test piece made the evaluation value (forgeability evaluation value%).

  Table 2 shows the evaluation results of abrasiveness, fatigue characteristics and cold forgeability together with the observation results of TiN in steel. No. in Table 1 And No. 2 in Table 2. Are the same as those in Table 2. The test results of No. 1 are shown in Table 1. It is assumed that steel having a component composition of 1 is used.

  From Table 2, it was confirmed that the steels of the present invention (Examples 1 to 17) had excellent abrasiveness, fatigue characteristics, and cold forgeability. On the other hand, in Comparative Example 1, C is outside the range of the present invention and the fatigue strength is low. In Comparative Example 2, C is high outside the range of the present invention, and cold forgeability is low.

  In Comparative Examples 3, 5, 7, 9, and 10, Ti is small outside the scope of the present invention, TiN is insufficient, polishing becomes over-grinding, and polishing properties are poor. In Comparative Examples 4, 6, 8, 11, and 12, the TiN is coarse and the amount of grinding at the time of polishing is insufficient, resulting in poor polishability.

  In Comparative Examples 13 to 20, the amount of alloy components added is large outside the range of the present invention, and the cold forgeability is poor. Comparative Example 14 has a small amount of Cr outside the range of the present invention and is inferior in fatigue strength. In Comparative Examples 21 and 22, TiN in the steel is coarse, the polishing amount is insufficient, the polishing property is inferior, and the fatigue strength is low.

It is a schematic diagram explaining a cold forgeability evaluation test, (a) is a test method, (b) is a figure which shows the generation | occurrence | production state of a crack.

Claims (2)

In mass%, C: 0.6 to 1.5%, Mn: 0.2 to 1.5%, Si: 0.05 to 1.2%, Cr: 0.5 to 2.5%, Ti: 0.002 to 0.020%, N: 0.008% or less, Al: 0.01 to 0.03%, Ti / N ≦ 3.4, remainder Fe and unavoidable impurities, steel composition A bearing steel having excellent polishing properties, wherein the maximum diameter of TiN is 10 μm or less and the number of TiN is 0.25 or more per 1 mm ² . In addition to the component composition according to claim 1, Cu: 0.2% or less, Ni: 0.2% or less, Mo: 0.1% or less, or one or two or more of polishing properties, Excellent bearing steel.

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