JP5145774B2 - Method for manufacturing rolling bearing component and rolling bearing - Google Patents

Description

  The present invention relates to a method for manufacturing a rolling bearing component.

  Rolling bearings used in steel facilities are often used in harsh environments such as under heavy loads and high surface pressures. Therefore, it is required to increase the rolling fatigue life even in such harsh environments. ing. Therefore, when manufacturing bearing rings and rolling elements for rolling bearings used in such a harsh environment, case hardening steel such as SNC815 to which Ni (nickel) or Mo (molybdenum) is added is used, and shear stress is applied. Carburizing or carbonitriding for a long time to obtain an effective hardened layer depth that can withstand, and quenching and tempering are performed.

  Moreover, in patent document 1, in order to suppress the surface damage which arises in severe environments, such as under a heavy load, high surface pressure, and foreign material mixing, at least one of an inner ring, an outer ring, and a rolling element is a unit. It is made of steel in which the number and size of non-metallic inclusions (oxide inclusions) existing per area is specified, and the amount of retained austenite of the surface layer is 20% by volume or more and 45% by volume or less. Has been proposed.

Furthermore, in Patent Document 2, a bearing that constitutes a rolling member is used to suppress internal damage originating from non-metallic inclusions that occur under severe environments such as high loads, high surface pressures, and clean environments. It has been proposed to define the number and size of non-metallic inclusions (oxide inclusions or Ti inclusions) present per unit area or unit volume in steel.
Non-Patent Document 1 discloses a method (Hall-Williamson) for determining the local strain ε _hkl from the lattice constant and half-value width of each peak of (110) (211) (220) of αFe obtained by X-ray diffraction. Method).
Japanese Patent Laid-Open No. 6-145883 JP 2004-84869 A G. K. Williamson and W.W. H. Hall; Acta Metal 1 (1953) 22

  In recent years, rolling mill rolling bearings used particularly in high-load environments not only cause surface damage due to contamination but also form damage different from internal damage caused by non-metallic inclusions. There is a problem that the rolling fatigue life cannot be made sufficiently long only by the definition of the object.

That is, in a large rolling bearing having an outer diameter of 200 mm or more, since the thickness is formed to be large, the depth of the completely hardened layer by quenching becomes insufficient, and fatigue strength and life due to incomplete quenching are reduced. There is concern about the decline. In particular, when carburized or carbonitrided, there is a residual tensile stress at the boundary (cut) between the hardened layer and the core, and therefore there is an incompletely quenched structure (bainite) that is the source of stress concentration in this part. Then, there is a possibility that destruction will occur starting from that part.
An object of the present invention is to increase the rolling fatigue life of a rolling bearing in which a unique form of damage occurs, such as a rolling mill rolling bearing.

In order to solve the above-mentioned problems, the present invention comprises a structure comprising an inner ring, an outer ring, or a rolling element of a rolling bearing by performing a heat treatment including carburizing or carbonitriding after a steel material is processed into a predetermined shape. In the method for producing a member, a method for producing a rolling bearing constituent member is provided, wherein the following constitutions (a) and (b) are satisfied.

[Configuration (a)]
Carbon content [C] is 0.1% by mass or more and 0.2% by mass or less, chromium content [Cr] is 0.5% by mass or more and 1.5% by mass or less, and silicon content [Si] is 0.1% by mass. Mass% to 0.5 mass%, manganese content [Mn] is 0.2 mass% to 0.6 mass%, molybdenum content [Mo] is 0.1 mass% to 0.5 mass%, A steel having a nickel content [Ni] of 3.0% by mass or more and 5.0% by mass or less and the balance of iron (Fe) and inevitable impurities is used.

[Configuration (b)]
The following is a variable with the former austenite maximum particle size d _M (μm) of the core, the average value M and standard deviation σ of the core hardness (Vickers hardness: Hv), and the local strain ε _hkl corresponding to the dislocation density as follows: Heat treatment including carburizing or carbonitriding is performed so that the material parameter Y represented by the formula (1) is 600 or more.
Y = (0.4 (ε _hkl −0.748) + 1.2 + 3 / √d _M ) (M−4σ) (1)

[Configuration (a)]
The reason for limiting [C] to be 0.1 mass% or more and 0.2 mass% or less is as follows.
Carbon is an element that strengthens steel by converting the structure to martensite. In the method of the present invention, the surface is hardened by carburizing or carbonitriding, but the carbon content is set to 0.1% by mass or more in order to give the core the necessary strength. However, if the carbon content exceeds 0.2% by mass, the toughness of the core is impaired.

The reason for limiting [Cr] to 0.5 mass% or more and 1.5 mass% or less is as follows.
Chromium is an element that improves the hardenability and temper softening resistance by solid solution in the matrix and also has the effect of improving the rolling fatigue life. It also has the effect of improving wear resistance by forming fine carbides and carbonitrides. When the chromium content is less than 0.5% by mass, these effects are not substantially obtained. However, if the chromium content exceeds 1.5% by mass, a passive film is formed on the surface, which may inhibit carburization.

The reason for limiting [Si] to 0.1 mass% or more and 0.5 mass% or less is as follows.
Silicon is an element that acts as a deoxidizer and a desulfurizer during steelmaking. If the silicon content is less than 0.1% by mass, the effect is not substantially obtained. However, when the silicon content exceeds 0.5% by mass, workability such as forgeability and machinability of the material is deteriorated.
The reason for limiting [Mn] to 0.2 mass% or more and 0.6 mass% or less is as follows.
Manganese is an element that acts as a deoxidizing agent and a desulfurizing agent during steelmaking and improves the hardenability by dissolving in a matrix. When the manganese content is less than 0.2% by mass, these effects are not substantially obtained. Preferably it is 0.3 mass% or more. However, if the manganese content exceeds 0.6% by mass, coarse non-metallic inclusions that cause a reduction in rolling fatigue life are likely to be generated, and workability such as material forgeability and machinability. Decreases.

The reason for limiting [Mo] to be 0.1% by mass or more and 0.5% by mass or less is as follows.
Molybdenum is an element that has the effect of improving the hardenability of steel and the strength and toughness after tempering. When the molybdenum content is less than 0.1% by mass, these effects are not substantially obtained. However, when the molybdenum content exceeds 0.5% by mass, a decrease in hot workability, an increase in manufacturing cost, and the like become problems.

The reason why [Ni] is 3.0% by mass or more and 5.0% by mass or less is as follows.
Nickel is an element having an effect of improving the hardenability of steel and the toughness after tempering. If the nickel content is less than 3.0% by mass, the effect is not substantially obtained. However, if the nickel content exceeds 5.0% by mass, problems such as a decrease in hot workability and an increase in manufacturing cost become a problem.
In order to reduce oxide-based non-metallic inclusions, the oxygen content in the steel used is preferably 12 ppm or less, and more preferably 9 ppm or less.

[Configuration (b)]
Y = (0.4 (ε _hkl −0.748) + 1.2 + 3 / √d _M ) (M−4σ) (1)
Equation (1) is expressed as follows: old austenite maximum particle size d _M (μm) of core, average value M and standard deviation σ of core hardness (Vickers hardness: Hv), local strain ε corresponding to dislocation density _hkl is a variable.
The structure of steel constituting the bearing component is generally in a state where crystal grains of various sizes are mixed due to the influence of forging and the influence of composition fluctuation. Fatigue fracture occurs starting from the weakest part in the applied range. Therefore, in the steel constituting the bearing constituent member, it is assumed that the largest austenite grains martensite by quenching are the starting point of fatigue failure. Further, it is known that the material strength is proportional to the inverse of the 1/2 power of the maximum value (d _M ) of the prior austenite grain size (1 / √d _M ).

The maximum value (d _M ) of the prior austenite grain size is a value obtained by the following method. First, after performing a predetermined heat treatment on a steel sample, the prior austenite grain boundaries were exposed, and 30 ranges with 100 mm ² as one observation range were observed with a microscope, and the maximum crystal grain size in each range. Is measured from the average value of the major axis and the minor axis of the prior austenite grain size. Next, an extreme value statistical graph is created from the obtained measured values, the maximum particle size predicted when the area is 15000 mm ² is calculated, and this is calculated as the maximum value (d _M ) of the prior austenite particle size. To do.
As the local strain ε _hkl , a value obtained by the method described in Non-Patent Document 1 (Hall-Williamson method) is used.

The average value M and the standard deviation σ of the core hardness (Vickers hardness: Hv) are calculated from the hardness measured at 50 points of the same sample using a micro Vickers hardness tester with a load of 4.9 N added. . It is known that “M-4σ” is proportional to the fatigue strength.
And it heat-treats so that the material parameter Y represented by (1) Formula may be 600 or more, and produces a rolling bearing structural member, thereby rolling surface (the raceway surface of the bearing ring and the rolling surface of the rolling element). An incompletely quenched structure (bainite) that becomes a stress concentration source hardly occurs at the boundary between the hardened layer and the core. Therefore, the internal fracture starting from the boundary is less likely to occur in the rolling bearing assembled by using the rolling bearing component produced by this method.

According to the method for manufacturing a rolling bearing constituent member of the present invention, the destruction of the internal starting point due to the incompletely quenched structure of the obtained member (the inner ring, outer ring, or rolling element of the rolling bearing) is suppressed. Therefore, the rolling fatigue life of the rolling mill rolling bearing obtained by applying this method can be extended.

Hereinafter, embodiments of the present invention will be described.
By finely adjusting the alloy composition of the improved SNCM815 steel, materials made of steel with varying ε _hkl and “M-4σ” etc. are prepared, and each material is a cylindrical roller bearing (inner diameter: NU228). 140 mm, outer diameter: 250 mm, width: 42 mm) The inner ring, outer ring, and cylindrical roller (rolling element) were molded by hot forging. Hot forging was performed at 1000 to 1250 ° C.
The average content of alloy components of the improved SNCM815 steel is as follows: the carbon content [C] is 0.2% by mass, the silicon content [Si] is 0.3% by mass, and the manganese content [Mn] is 0. 0.5% by mass, chromium content [Cr] is 1.2% by mass, molybdenum content [Mo] is 0.1% by mass, and nickel content [Ni] is 3.2% by mass. is there.

Next, heat treatment was performed according to the following procedure.
First, the normalizing process which cools after hold | maintaining at 750-950 degreeC for 1 hour was performed. Next, as a carburizing treatment, after holding in an RX gas atmosphere at a temperature of 800 to 1050 ° C. for 10 to 90 hours, cooling is performed at a cooling rate corresponding to the crystal grain size. In order to give the shear stress required for the large-sized bearing for rolling mills (equivalent to a nominal number of 600 RV) by this carburizing treatment, the carbon content of the surface layer portion is 0.7 to 1.0% by mass, and the carburized depth is 1000. It was made to become -2000 micrometers.

Next, as an annealing treatment, it is allowed to cool after being held at 500 to 700 ° C. for 1 to 3 hours. Next, as a quenching treatment, oil cooling is performed after holding at 750 to 880 ° C. for 1 to 3 hours. Next, as a tempering treatment, it is allowed to cool after being held at 150 to 300 ° C. for 2 hours.
By changing the conditions of this heat treatment, as shown in Table 1, the old austenite maximum particle diameter d _M of the core part, the average value M of Vickers hardness, the standard deviation σ, and the local strain ε _hkl are obtained for each sample number. Different inner rings, outer rings and cylindrical rollers were obtained.

A cylindrical roller bearing is assembled using the obtained inner ring, outer ring, and cylindrical roller, and a rotational life test is performed under the conditions of radial load: P / C = 0.6, rotational speed: 1000 min ⁻¹ , lubricant: Ro68. It was. Judgment of the life was performed by detecting an increase in vibration value accompanying peeling. From the L10 life of each sample, a relative value with the L10 life of No. 15 with Y = 557 as “1” was calculated. The results are also shown in Table 1.

  Moreover, the raw material which consists of steel of the same composition as sample No. 1-17 was heat-processed by the same method as the above with respect to each test piece as a test piece of 150 mm x 200 mm x thickness 40mm for every sample No. . For each of the obtained test pieces, a rotary bending test piece (minimum diameter of 8 mm) was cut out from the central portion of the thickness and subjected to a rotary bending test by the Ono type rotary bending test method. The maximum value of the stress that does not break even when loaded over 10,000 times was measured. The results are also shown in Table 1 below.

In addition, the Vickers hardness of the core portion was measured at a test load of 4.9 N at 50 locations in the 8 mm diameter region at the center of each obtained test piece, and the average value (M) and standard deviation (σ) were calculated. Asked. And M-4σ was calculated from these results. The results are also shown in Table 1 below.
In addition, test pieces for X-ray diffraction were similarly prepared, and each test piece was heat-treated by the same method as described above for each sample number. The local strain ε _hkl was determined by the Hall-Williamson method from the results of X-ray diffraction for each obtained test piece. The results are also shown in Table 1 below.

Further, “M-4σ” and Y were calculated from these results. These calculated values are also shown in Table 1 below.
A graph summarizing these results in the relationship between the fatigue limit and the material parameter Y is shown in FIG. A graph summarizing these results in the relationship between the relative value of the L10 life (rolling life ratio) and the material parameter Y is shown in FIG.

  As can be seen from FIG. 1, the fatigue limit and the material parameter Y are in a proportional relationship. As can be seen from FIG. 2, the life of the rolling bearing can be significantly increased by setting the material parameter Y to 600 or more.

It is the graph which put together the result of the test done in the embodiment in the relation between the fatigue limit and the material parameter Y. It is the graph which put together the result of the test done in the embodiment in the relation between rolling life ratio and material parameter Y.

Claims (2)

In a method of manufacturing a constituent member made of an inner ring, an outer ring, or a rolling element of a rolling bearing by performing a heat treatment including carburizing or carbonitriding after processing a material made of steel into a predetermined shape,
Carbon content [C] is 0.1% by mass or more and 0.2% by mass or less, chromium content [Cr] is 0.5% by mass or more and 1.5% by mass or less, and silicon content [Si] is 0.1% by mass. Mass% to 0.5 mass%, manganese content [Mn] is 0.2 mass% to 0.6 mass%, molybdenum content [Mo] is 0.1 mass% to 0.5 mass%, Using nickel steel with a nickel content [Ni] of 3.0% by mass or more and 5.0% by mass or less, the balance being iron (Fe) and inevitable impurities,
The following is a variable with the former austenite maximum particle size d _M (μm) of the core, the average value M and standard deviation σ of the core hardness (Vickers hardness: Hv), and the local strain ε _hkl corresponding to the dislocation density as follows: A method for manufacturing a rolling bearing constituent member, comprising performing a heat treatment including carburizing or carbonitriding so that the material parameter Y represented by the formula (1) is 600 or more.
Y = (0.4 (ε _hkl −0.748) + 1.2 + 3 / √d _M ) (M−4σ) (1)   The rolling bearing provided with the inner ring | wheel, the outer ring | wheel, or rolling element obtained by the method of Claim 1.

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