JP5463662B2 - Bearing steel excellent in rolling fatigue characteristics and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a bearing steel suitable for machine parts such as automobile parts and industrial machine parts, and particularly suitable for bearing parts and excellent in workability and rolling fatigue characteristics suitable for large parts that are difficult to be heat-treated. And its manufacturing method. The bearing parts here are parts constituting bearings such as bearing inner and outer rings, bearing balls, bearing rollers and needles.

For bearing parts, for example, steel such as SUJ2 and SUJ3 specified in JIS G4805 is used to form a hard martensite structure by quenching and tempering after forming into a predetermined shape by cold forging or hot forging. It is customary to do this.
By the way, with the widespread use of wind power generators, demand for large-sized bearing parts is increasing. Due to the increase in the size of such parts, industrially, heat treatment distortion and machinability at the time of quenching become problems as compared with conventional ones, and it is desired to develop a bearing steel with less quenching distortion and excellent machinability. Yes. Generally, a bainite structure is used as a high-strength structure next to the martensite structure, but the strength is lower than martensite obtained by quenching and tempering treatment, so it is unsuitable for bearing applications and has been put to practical use. Not in.

Here, Patent Document 1 proposes that bainite is generated at an extremely low temperature as compared with the prior art to achieve a high strength that was not obtained conventionally. This improved bainite steel is obtained by homogenizing steel containing 0.6 to 1.1% carbon at 1150 ° C. for 24 hours, air cooling, and further heating to a temperature between 900 to 1000 ° C., followed by 190 to 260 ° C. Steel having a bainite structure of at least 50% obtained by isothermal transformation at a temperature between 1 to 3 weeks. However, the steel has been focused on obtaining high strength and has not been considered for workability, so it has been difficult to industrially use the steel. Moreover, the knowledge about the structure | tissue which can obtain high intensity | strength is inadequate, and the relationship between the structure factor for obtaining the intensity | strength required for bearing components and manufacturing conditions was not clear.
Japanese Patent No. 3751250

  Therefore, the present invention has been developed in view of the above-described present situation, and an object thereof is to propose a bainite-type bearing steel that does not generate heat treatment distortion and that is suitable for application to a large part, and a manufacturing method thereof. .

Now, the inventors have studied the optimum steel structure for the purpose of obtaining a bearing part that achieves the strength required for the bearing part as described above and that is excellent in machinability. It came to guide.
That is, the gist configuration of the present invention is as follows. (1) By mass%
C: 0.6% or more and 1.2% or less,
Si: 0.8% or more and less than 1.5%
Mn: 0.5% to 3.5%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.005% or more and 0.050% or less,
Cr: 0.1% to 2.0% and
O: 0.0012% or less
And the balance has a component consisting of Fe and inevitable impurities,
Moreover, an average width of a is 60nm or less, containing a plate-shaped ferrite volume fraction is 50% or more, and a residual austenite generated in layers between the remainder canvas ferrite, that has a tissue Bearing steel with excellent rolling fatigue characteristics.

( 2 ) In the bearing steel according to ( 1 ),
Mo: 1.0% or less,
Cu: 1.0% or less and
Ni: Bearing steel excellent in rolling fatigue characteristics characterized by containing one or more of 1.0% or less.

( 3 ) In the bearing steel according to ( 1 ) or ( 2 ),
Ti: 0.2% or less,
V: 0.2% or less and
Nb: Bearing steel excellent in rolling fatigue characteristics characterized by containing one or more of 0.1% or less.

( 4 ) Mass% C: 0.6% or more and 1.2% or less,
Si: 0.8% or more and less than 1.5%
Mn: 0.5% to 3.5%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.005% or more and 0.050% or less,
A steel containing Cr: 0.1% or more and 2.0% or less and O: 0.0012% or less, the balance of which is composed of Fe and inevitable impurities is heated to 800 ° C. or higher, and then the cooling rate V (° C./s) and The martensitic transformation obtained by the following equation (2) is performed by cooling to the isothermal transformation temperature so that the relationship with the critical cooling rate Vcr (° C./s) obtained by the equation (1) satisfies V> Vcr. A method for producing a bearing steel having excellent rolling fatigue characteristics, characterized in that one or more steps of isothermal transformation treatment are performed at (Ms + 10) to 250 ° C. for a temperature (Ms) of 20 ksec or more.
Record
Vcr = 0.32−0.54C% + 0.018Si% + 1.29Mn% + 0.61Ni% + 0.85Cr% + 0.69Cu% + 4.9Mo% (1)
Ms (° C) = 561−474C% −33Mn% −17Cr% −17Ni% −21Mo% (2)

( 5 ) In the above ( 4 ), further
Mo: 1.0% or less,
Cu: 1.0% or less and
Ni: A method for producing a bearing steel excellent in rolling fatigue characteristics, comprising one or more of 1.0% or less.

( 6 ) In the above ( 4 ) or ( 5 ),
Ti: 0.2% or less,
V: 0.2% or less and
Nb: A method for producing a bearing steel excellent in rolling fatigue characteristics, comprising one or more of 0.1% or less.

  By using the bearing steel of the present invention, it is possible to stably obtain bainite-type bearing parts and bearings excellent in rolling fatigue characteristics.

Hereinafter, the present invention will be specifically described.
That is, the bearing steel of the present invention contains plate-like ferrite having an average width of 60 nm or less and a volume fraction of 50% or more, and the balance is mainly composed of retained austenite that is generated in layers between the ferrites. It is characterized by having an organization.
Therefore, first, an explanation will be given from experimental results that have led to the finding that the inclusion of 50% or more of plate-like ferrite having an average width of 60 nm or less satisfies the characteristics as bearing steel.

  Now, in order to form a martensite structure often used as bearing steel, it is necessary to rapidly cool from the austenite region to below the martensite transformation start point (Ms point) at a cooling rate at which ferrite, pearlite and bainite are not generated. is there. However, in the case of a large part, it is difficult to cool the material uniformly, and heat treatment distortion due to a temperature difference in the part occurs. Further, since the martensitic transformation is accompanied by a large volume expansion, there is a problem that a large internal stress is generated due to a shift in the timing of the martensitic transformation for each part of the part, and cracks are likely to occur.

  On the other hand, the bainite structure is obtained in the temperature range above the Ms point, and in particular, when bainite is generated by isothermal treatment, the temperature of the material is uniform, so there is no distortion or cracking due to the temperature difference between parts. . Therefore, attention was focused on the bainite structure as a means for solving the above-described problems.

  In the bainite transformation, supersaturated ferrite of C is generated in a shearing manner, and then C diffuses into surrounding austenite, resulting in concentration of C in the austenite. That is, after the bainite transformation, a mixed structure of ferrite and retained austenite is obtained. In order to refine the ferrite formed in this way, (i) to lower the isothermal transformation temperature, (ii) to suppress the formation of carbides in order to maintain the microstructure, two-phase structure of retained austenite and ferrite It is necessary to. In order to make it difficult to form carbide, Si addition of 0.8% or more is effective. On the other hand, if Si is added excessively, the machinability is remarkably lowered, so it is necessary to limit it to less than 1.5%.

  Next, the result of an experimental study on the form of the structure having conditions as bearing steel will be shown. C: 0.5-1.5 mass%, Si: 0.5-2.0 mass%, Mn: 0.25-4.0 mass%, P: 0.050 mass% or less, Al: 1 mass% or less, Cr: 3.0 mass% or less, O: 0.0020 mass% Steel having various composition compositions in the following ranges and steel according to JIS SUJ2, which is a comparative steel, were melted in a vacuum melting furnace in an amount of 100 kg, and this was formed into a 30 mmφ bar steel by hot forging. A radial rolling fatigue test piece having a diameter of 12.5 mm and a length of 22 mm from a 1/4 diameter position (D / 4 part) of the steel bar was produced by rough machining. The test steel was heated and held at 850 ° C for 30 minutes, then kept at 170 ° C to 300 ° C at various holding temperatures for 20 ksec or more, cooled to room temperature, and finished to a 12 mmφ and 22 mm long test piece by finishing. It was. The SUJ2 steel was heated and held at 850 ° C. for 30 minutes, then quenched in oil at 60 ° C., tempered at 170 ° C. for 30 minutes, and then finished.

  The rolling life of the test piece thus obtained was evaluated. The rolling life was evaluated in the following manner. Using a radial type rolling fatigue tester, 20 tests were conducted under the conditions of Hertz stress 5884 MPa and rotation speed 46000 cpm, and the cumulative failure probability 10% obtained by Weibull plotting of each specimen life The lifespan (B10 lifespan) was evaluated.

  Moreover, the plate width of the plate-like ferrite was investigated for the structure of each test steel. Here, the plate-like ferrite is a ferrite formed in a shearing manner by bainite transformation, and is characterized by being finer and having a higher dislocation density than ferrite obtained by ordinary diffusion transformation.

  The reason why the plate width in the plate-like ferrite is focused is that the finer the plate width, the higher the strength.

  Since the plate width of ferrite is very fine, in the measurement, a test piece processed into a thin film and electropolished is observed with a transmission electron microscope at five fields of view at 30,000 times, and the average value is obtained. It was measured. Furthermore, the ferrite fraction was measured by the X-ray method. These results are shown in FIG.

  From FIG. 1, it is clear that there is a region where a B10 life equal to or greater than that of SUJ2 steel can be obtained even though the steel structure is not conventional tempered martensite. That is, it has been found that a B10 life equal to or greater than that of SUJ2 steel can be obtained by setting the steel structure to an average ferrite width of 60 nm or less and a ferrite fraction of 50% or more.

Balance other than the ferrite structure is composed of residual austenite to produce a layer between ferrites. That is, as a result of C discharging from the C supersaturated ferrite generated in a shearing manner to austenite by the bainite transformation, the C concentration in the austenite increases. When the C concentration in austenite increases, cementite usually precipitates, but since the formation of carbides is suppressed by the increase in Si content, the structure is composed of two phases of ferrite and retained austenite. In order to obtain a B10 life equal to or longer than that of SUJ2 steel, a two-phase structure of these ferrite and retained austenite is desirable. In the unlikely event that the specimen steel is cooled before the bainite transformation completely stops during the isothermal holding, some martensite structure will be mixed, but if these martensite structures are 15% or less of the entire structure, It has almost the same characteristics as the case of a two-phase structure of ferrite and retained austenite without causing fire cracking.

Next, a suitable component composition for obtaining the above structure will be described in detail. In addition, unless otherwise indicated, the "%" display regarding the component shown below shall represent the mass%.
C: 0.6-1.2%
C has an effect of lowering the Bs point and needs to be contained in an amount of 0.6% or more. C has the effect of concentrating to austenite after bainite transformation and stabilizing the retained austenite. However, if it exceeds 1.2%, the machinability is greatly reduced, so it is preferably in the range of 0.6 to 1.2%.

Si: 0.8% or more and less than 1.5%
Si functions to suppress the formation of carbides and stabilize retained austenite. In order to obtain this effect, it is necessary to contain 0.8% or more. However, if excessively added, the machinability is remarkably lowered, so 0.8% or more and less than 1.5% is preferable.

Mn: 0.5-3.5%
Mn has the effect of lowering the Bs point. If it is less than 0.5%, the effect is poor, and if it exceeds 3.5%, the machinability is greatly reduced. Therefore, the content is preferably in the range of 0.5 to 3.5%.

P: 0.030% or less P is an impurity element, and if it exceeds 0.030%, the rolling life decreases, so 0.030% or less is preferable.

S: 0.030% or less S is an impurity element, and if it exceeds 0.030%, the rolling life is reduced. Therefore, the content is preferably 0.030% or less.

Al: 0.005 to 0.050%
Al is an element necessary as a deoxidizing element. Less than 0.005%, the effect is poor, 0.050%
Since the effect is saturated in the above, it was made 0.005 to 0.050% of range.

Cr: 0.1-2.0%
Cr, like Mn, has the effect of lowering the Bs point. If it is less than 0.1%, the effect is poor, and if it exceeds 2.0%, the retained austenite is destabilized, so the range of 0.1 to 2.0% is preferable.

O: 0.0012% or less O is preferably 0.0012% or less in order to reduce the rolling life as non-metallic inclusions.

Furthermore, in addition to the above basic components, the following elements can be added.
Mo: 1.0% or less
Mo has an effect of lowering the Bs point, and is preferably added at 0.1% or more. However, if it exceeds 1.0%, the machinability is reduced, so 1.0% or less is preferable.

Cu: 1.0% or less
Cu has the function of strengthening bainite as a solid solution strengthening element, and is preferably added at 0.2% or more, but if added over 1.0%, cracking occurs during hot working, so it should be 1.0% or less. Is preferred.

Ni: 1.0% or less
Ni has the effect of lowering the Bs point, and is preferably added at 0.2% or more. However, if it exceeds 1.0%, the machinability is lowered, so 1.0% or less is preferable.

Ti: 0.2% or less
Ti, as an oxide or nitride, has the effect of suppressing the coarsening of austenite grains during heating, and combines with C and N to act as a precipitation strengthening element. In order to obtain this effect, 0.005% or more, more preferably 0.05% or more is added. However, even if added over 0.2%, the effect is saturated, so 0.2% or less is preferable.

V: 0.2% or less V is combined with C and N in the steel and acts as a precipitation strengthening element, so 0.05% or more is preferably added. However, even if added over 0.2%, the effect is saturated, so 0.2% or less is preferable.

Nb: 0.1% or less
Nb combines with C and N in the steel and acts as a precipitation strengthening element, so 0.05% or more is preferably added. However, even if added over 0.1%, the effect is saturated, so 0.1% or less is preferable.

Next, we examined the conditions for obtaining tissue fine ferrite and the remainder being residual austenite. In order to obtain this structure, it is necessary to suppress ferrite and pearlite generated in the cooling process after heating to 800 ° C. or higher and then austenite. The critical cooling rate V at which this can be achieved is expressed by the following formula (1): It was revealed that this is possible when V> Vcr.
Vcr = 0.32−0.54C% + 0.018Si% + 1.29Mn% + 0.61Ni% + 0.85Cr% + 0.69Cu% + 4.9Mo% ... (1)

Cooling under the condition that the cooling rate V is V> Vcr is stopped in a temperature range of (Ms + 10) to 250 ° C. with respect to the martensite transformation temperature Ms obtained by the following equation (2), and thereafter, this temperature One-stage or multiple-stage isothermal transformation treatment is performed in the range.
Ms (° C) = 561−474C% −33Mn% −17Cr% −17Ni% −21Mo% (2)
When the temperature of the isothermal transformation treatment is equal to or lower than the Ms point, martensite is generated and the problem of fire cracking occurs. Therefore, in the present invention, it is intended to provide a bearing steel suitable for large parts. It is important to prevent burning cracks. For this reason, it is necessary to set (Ms + 10) ° C. as the lower limit of the isothermal transformation temperature so that martensite is not generated or even if it is generated, the fraction can be 15% or less. On the other hand, when the constant temperature transformation treatment temperature is increased, the width of the plate-like ferrite is increased. When the isothermal transformation temperature exceeds 250 ° C., the average width of the plate-like ferrite exceeds 60 nm, so that the effect of improving the rolling fatigue life cannot be obtained. Therefore, the isothermal transformation treatment is performed in the range of (Ms + 10) to 250 ° C.

The constant temperature transformation treatment is performed for 20 ksec (20 × 10 ³ sec) or more. When the isothermal transformation time is short, martensite is generated at 15% or more. When the isothermal transformation process is performed in two or more stages, the total processing time may be 20 ksec or more.

A 100 kg vacuum steel ingot having the composition shown in Table 1 was melted, homogenized at 1250 ° C. for 24 hours, hot forged at a temperature of 900 ° C. or higher, and processed into a 30 mmφ round bar. A radial rolling fatigue test piece of 12.5mmφ and 22mm length was roughly machined from D / 4 part of the obtained steel bar, heated at 850 ° C for 30 minutes, then cooled at various cooling rates shown in Table 2 and kept at constant temperature. Transformation processing was performed. After the isothermal transformation treatment, it was once cooled to room temperature, and finished into a radial test piece having a diameter of 12 mm and a length of 22 mm.
On the other hand, SUJ2 steel (steel No. 23) was heated and held at 850 ° C for 30 minutes, then quenched with oil at 60 ° C, and then tempered at 170 ° C for 30 minutes. Finishing was performed.

  The rolling life was evaluated based on the ratio of the BIO life by radial test to the B10 life of SUJ2 steel. In the radial test, 20 tests were performed at Hertz stress of 5884 MPa and a rotational speed of 46000 cpm to obtain the B10 life.

  In addition, the average width of the plate-like ferrite of the test steel is the above-mentioned average width of 5 fields at a magnification of 30,000 times using a transmission electron microscope for a sample processed into a thin film and electropolished. The measurement was made accordingly. On the other hand, the ferrite fraction was measured by the X-ray method.

Furthermore, in the machinability test, a heat treatment similar to that of a radial test piece was performed with a round bar of 30 mmφ and 400 mm length, and the tool life obtained by the peripheral turning test was better than SUJ2, ○, Was evaluated as x. The peripheral turning test was performed using a carbide tool P10, with a turning speed of 200 m / min, a feed of 0.15 mm / rev, a cut of 1 mm, and lubrication.
The tool life was evaluated by the time until the flank wear of the tool reached 0.2 mm.
The results of each evaluation are also shown in Table 2.

  From Table 2, symbols 1 to 13 are examples of the present invention, and showed rolling fatigue characteristics and machinability equivalent to or better than SUJ2. On the other hand, symbols 14 to 25 are all outside the scope of the present invention, and either or both of rolling fatigue characteristics and machinability were inferior to SUJ2 steel.

It is a graph which shows the influence which the average width of a plate-shaped ferrite and a ferrite fraction have on B10 life.

Claims (6)

In mass%
C: 0.6% or more and 1.2% or less,
Si: 0.8% or more and less than 1.5%
Mn: 0.5% to 3.5%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.005% or more and 0.050% or less,
Cr: 0.1% to 2.0% and
O: 0.0012% or less
And the balance has a component consisting of Fe and inevitable impurities,
Moreover, an average width of a is 60nm or less, containing a plate-shaped ferrite volume fraction is 50% or more, and a residual austenite generated in layers between the remainder canvas ferrite, that has a tissue Bearing steel with excellent rolling fatigue characteristics. The bearing steel according to claim 1, further comprising:
Mo: 1.0% or less,
Cu: 1.0% or less and
Ni: 1.0% or less
Bearing steel excellent in rolling fatigue characteristics, characterized by containing one or more of the above . The bearing steel according to claim 1 , further comprising:
Ti: 0.2% or less,
V: 0.2% or less and
Nb: Bearing steel excellent in rolling fatigue characteristics characterized by containing one or more of 0.1% or less . In mass%
C: 0.6% or more and 1.2% or less,
Si: 0.8% or more and less than 1.5%
Mn: 0.5% to 3.5%,
P: 0.030% or less,
S: 0.030% or less,
Al: 0.005% or more and 0.050% or less,
Cr: 0.1% to 2.0% and
O: 0.0012% or less
And the balance of Fe and the inevitable impurities are heated to 800 ° C. or higher, and then the cooling rate V (° C./s) and the critical cooling rate Vcr (° C. / s) is cooled to a constant temperature transformation treatment temperature so that V> Vcr, and then (Ms + 10) to 250 ° C. with respect to the martensitic transformation temperature (Ms) obtained by the following equation (2) A method for producing a bearing steel having excellent rolling fatigue characteristics, characterized by performing one-stage or multiple-stage isothermal transformation treatment for 20 ksec or more .
Record
Vcr = 0.32−0.54C% + 0.018Si% + 1.29Mn% + 0.61Ni% + 0.85Cr% + 0.69Cu% + 4.9Mo% (1)
Ms (° C) = 561−474C% −33Mn% −17Cr% −17Ni% −21Mo% (2) In claim 4, further:
Mo: 1.0% or less,
Cu: 1.0% or less and
Ni: 1.0% or less
The manufacturing method of the bearing steel excellent in the rolling fatigue characteristic characterized by containing 1 type, or 2 or more types of these . In claim 4 or 5, further
Ti: 0.2% or less,
V: 0.2% or less and
A manufacturing method of bearing steel excellent in rolling fatigue characteristics, characterized by containing one or more of Nb: 0.1% or less .

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