JP2007262449A - Steel for rolling parts, and rolling parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for rolling parts, which extends the life of parts such as a bearing for use in an automotive engine auxiliary machine, by preventing the structure of a steel material of parts such as a bearing from whitening, and to provide the rolling parts. <P>SOLUTION: A base steel includes, by mass%, 0.50-1.1% C, 0.1-1.2% Si, 0.2-1.5% Mn, 3.0-10.0% Cr, 0.02-0.05% N, 0.03% or less P, 0.01% or less S, 0.005-0.05% Al, 0.005% or less Ti, 0.0015% or less O, and the balance Fe with unavoidable impurities. The amount of retained austenite in the steel for the rolling parts is 8 to 30% after the base steel has been quenched at a quenching temperature of 900 to 1,080°C, and tempered at 150 to 230°C; and is decreased by 3% or less when having been held at 300°C afterward. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The invention of this application includes a steel for rolling parts used for an alternator for an automotive engine electrical accessory, an electromagnetic clutch, an intermediate pulley, a compressor for a car air conditioner, a water pump, a belt type CVT, a differential, and the like, and this steel material. In particular, the present invention relates to a rolling part steel excellent in white structure resistance and a rolling part made of this steel material.

  Bearings used in alternators for automobile engine electrical auxiliary equipment, electromagnetic clutches, intermediate pulleys, compressors for car air conditioners, water pumps, and the like have been rotated at higher speeds as the size is reduced. For this reason, early peeling occurs in these bearings with a white texture change (white layer). This early peeling is said to occur when hydrogen generated by the decomposition of the lubricating oil due to the minute slip occurring on the rolling surface enters the bearing material.

  As a measure to improve the life of steel for rolling parts such as bearings against short-time delamination accompanied by white structure change, focusing on stress-induced diffusion of carbon atoms due to repeated stress loading due to vibration, carbon diffusion suppression and matrix A device for improving the strength has been proposed (see Patent Document 1). However, the present invention does not consider the improvement of fatigue strength under hydrogen intrusion and residual austenite as a countermeasure.

  Furthermore, as a measure for improving the life against short-time peeling accompanied with a white layer, there has been proposed one that prevents entry of hydrogen by forming an oxide film on the transfer surface with Cr (see Patent Document 2). However, this invention also does not consider the improvement of fatigue strength in an environment where hydrogen is present and residual austenite as a countermeasure.

  Furthermore, as a measure to improve the life against short-time exfoliation with white layer, the effect of pinning the structure change by carbides and carbonitrides, improving the grain boundary strength by refining grain boundaries, and delaying the structure change propagation speed. Has been proposed (see Patent Document 3). However, the invention of this application does not take into account the improvement of fatigue strength under hydrogen intrusion and retained austenite as a countermeasure.

  Furthermore, a rolling bearing in which the retained austenite is considered to be 15 to 45% by volume has been proposed (see Patent Document 4). However, in the invention of this application, it is necessary to perform a carbonitriding process as a heat treatment, and a long-time heat treatment is required. However, this also does not consider the stability of retained austenite.

Japanese Patent No. 2883460 Japanese Patent No. 3009254 JP 2005-147352 A JP 2005-314794 A

  In parts such as alternators for automobile engine auxiliary equipment, electromagnetic clutches, intermediate pulleys, compressors for car air conditioners, water pumps, belt type CVTs, differentials, etc. Premature detachment occurs with tissue changes. Accordingly, the problem to be solved by the present invention is to provide a rolling part steel that prevents this white structure change and extends the life of parts such as bearings, and a rolling part made of this steel material. .

  As described above, the early separation that occurs with changes in the white structure of bearings used for automobile engine accessories, etc., is caused by hydrogen generated by the decomposition of lubricating oil due to minute slips that occur on the rolling surface. It is said that it is generated by entering the material.

  Therefore, the inventor conducted various tests on the bearing material and investigated the test pieces, and found that there was a difference in the amount of retained austenite between the long-life material and the short-life material. That is, even if this retained austenite was the same amount before the test, the retained austenite transformed into martensite while undergoing rolling fatigue, and the amount of retained austenite was reduced. If martensite transformation occurs during use, it becomes brittle martensite that has not been tempered, and fatigue cracks can easily be generated therefrom. Therefore, it is important that the retained austenite does not decompose even when fatigued.

  Therefore, when the life was evaluated by using the amount of decrease in retained austenite after raising the temperature to 300 ° C. as an index for evaluating the stability of retained austenite, the decrease in retained austenite after raising the temperature to 300 ° C. was evaluated for long-life materials. I found that the amount was small. In other words, the amount of retained austenite in the material is the key to rolling fatigue life in an environment where hydrogen penetrates into the bearing material of rolling parts steel, and the retained austenite decomposes in a fatigue environment. In other words, it was found that the stability of retained austenite is important for early exfoliation accompanied by a change in white structure occurring under hydrogen intrusion.

  Furthermore, when a method for stabilizing retained austenite was studied, it was found that it was important to increase the amount of Cr and N in the material, and the means of the present invention was obtained.

  Furthermore, the steel for rolling parts of the present invention has a long life of the bearing by preventing white structure change by simple quenching and tempering treatment without using carburizing or carbonitriding treatment that requires long-time heat treatment. It has been found that since the steel for rolling parts can be obtained, the life can be improved without significantly increasing the cost.

  Therefore, the means of the present invention for solving the above-mentioned problem is that, in the invention of claim 1, in mass%, C: 0.50 to 1.1%, Si: 0.1 to 1.2%, Mn : 0.2 to 1.5%, Cr: 3.0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0. A steel containing 005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015%, the balance being Fe and inevitable impurities, tempering from a quenching temperature of 900 to 1080 ° C. and tempering to 150 to 230 ° C. It is a steel for rolling parts characterized in that the amount of residual austenite after is 8 to 30% and the amount of decrease in residual austenite is 3% or less when held at 300 ° C. thereafter.

  Furthermore, in invention of Claim 2, C: 0.50-1.1%, Si: 0.1-1.2%, Mn: 0.2-1.5%, Cr: 3. 0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, Contains O ≦ 0.0015%, and further contains one or more selected from Ni ≦ 2.0%, Mo ≦ 1.5%, V ≦ 1.0%, Nb ≦ 0.3%. When the remaining austenite amount is 8 to 30% after quenching from a quenching temperature of 900 to 1080 ° C. and tempering at 150 to 230 ° C., and then kept at 300 ° C. It is a steel for rolling parts, characterized in that the amount of reduction in retained austenite is 3% or less.

  Furthermore, in invention of Claim 3, C: 0.50-1.1%, Si: 0.1-1.2%, Mn: 0.2-1.5%, Cr: 3. 0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015%, made of steel consisting of the balance Fe and inevitable impurities, the amount of retained austenite after tempering at a quenching temperature of 900-1080 ° C. and 150-230 ° C. is 8-30%, Thereafter, the amount of reduction in retained austenite when kept at 300 ° C. is 3% or less.

  Furthermore, in invention of Claim 4, C: 0.50-1.1%, Si: 0.1-1.2%, Mn: 0.2-1.5%, Cr: 3. 0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, Contains O ≦ 0.0015%, and further contains one or more selected from Ni ≦ 2.0%, Mo ≦ 1.5%, V ≦ 1.0%, Nb ≦ 0.3%. The residual austenite is 8-30% after quenching from a quenching temperature of 900-1080 ° C. and tempering at 150-230 ° C., and then remaining at 300 ° C. The rolling part is characterized in that the amount of reduction of austenite is 3% or less.

  The reasons for limiting the components in the rolling part steel of the above means will be described below. In addition,% is the mass%.

C: 0.50 to 1.1%
C is an element necessary for securing hardness by quenching and tempering, and for this purpose, 0.50% or more is required. On the other hand, it is necessary to make it 1.1% or less in order to prevent giant carbides that cause a reduction in life. Therefore, C is 0.50 to 1.1%, preferably 0.60 to 0.90%.

Si: 0.1-1.2%
Si is an element necessary for improving hardenability and increasing temper softening resistance. For this purpose, 0.1% or more is required. On the other hand, even if it exceeds 1.2%, the effect is saturated and the workability is lowered. Therefore, Si is 0.1 to 1.2%, preferably 0.4 to 0.9%.

Mn: 0.2 to 1.5%
Mn is an element necessary for improving the hardenability and obtaining an appropriate amount of retained austenite, and for this purpose, 0.2% or more is necessary. On the other hand, if it exceeds 1.5% and the amount is too much, retained austenite becomes excessive. Therefore, Mn is set to 0.2 to 1.5%.

Cr: 3.0 to 10.0%
Cr is an element that improves hardenability and stabilizes retained austenite, and 3.0% or more is required for this purpose. On the other hand, when there is too much Cr, in order to prevent generation | occurrence | production of the huge coal which causes a life fall, it is necessary to make Cr 10.0% or less. Therefore, Cr is set to 3.0 to 10.0%, preferably 4.0 to 8.0%.

P: ≦ 0.03%
P is inevitably contained as an impurity, but is made 0.03% or less because it makes the grain boundary brittle and lowers impact resistance.

S: ≦ 0.01%
S is inevitably contained as an impurity, but is set to 0.01% or less because the rolling life is reduced by sulfide.

O ≦ 0.0015%
O is inevitably contained as an impurity, but is an element that forms inclusions and reduces the lifetime. Therefore, O is made 0.0015% or less.

Ti ≦ 0.005%
Ti is inevitably contained as an impurity, but is an element that forms inclusions and decreases the lifetime. Therefore, Ti is made 0.005% or less.

Al: 0.005 to 0.05%
Al is an element necessary for deoxidation and crystal grain size adjustment. For this purpose, Al is required to be 0.005% or more. However, excess Al results in large inclusions due to reoxidation and shortens the lifetime. Therefore, Al is made 0.05% or less.

N: 0.02 to 0.05%
N is an element that stabilizes retained austenite. For this purpose, N must be 0.02% or more. However, when N is added excessively, voids are formed in the steel and the strength is lowered. Therefore, N is set to 0.05% or less.

  Furthermore, the reasons for limiting the components of the selected element will be described.

Ni: ≦ 2.0%
Ni is an element that improves hardenability and improves toughness. However, if Ni exceeds 2.0%, the effect is saturated and the cost is increased. Therefore, Ni is set to 2.0% or less.

Mo ≦ 0.5%
Mo is an element that improves hardenability and improves fatigue strength. However, if Mo exceeds 0.5%, the effect is saturated and the cost is increased. Therefore, Mo is 0.5% or less.

V ≦ 1.0%
V is an element that improves hardenability and improves fatigue strength. However, when V exceeds 1.0%, the effect is saturated and the cost is increased. Therefore, V is set to 1.0% or less.

Nb ≦ 0.3%
Nb is an element that refines crystal grains and improves fatigue strength. However, if Nb exceeds 0.3%, the effect is saturated and the cost is increased. Therefore, Nb is set to 0.3% or less.

  Further, the steel comprising the above-mentioned components in the means of the present invention is quenched after being held at a quenching temperature of 900 to 1080 ° C. for 30 minutes, and the amount of retained austenite after tempering at 150 to 230 ° C. is limited to 8 to 30%. As described above, the presence of an appropriate amount of retained austenite relieves stress concentration around the inclusions and improves the life. If the amount of retained austenite is less than 8%, the effect is not obtained, and if it exceeds 30%, the hardness is decreased by a large amount of retained austenite and the life is shortened. In addition, in the case of using a normal baked steel such as SUJ2 of JIS standard, the life is reduced by decomposition during use, so the residual austenite is controlled to 15% or less. However, in the steel of the means of the present invention, Since the retained austenite is stable, it is quenched by holding at a quenching temperature of 900 to 1080 ° C. for 30 minutes, and tempered to 150 to 230 ° C., and then kept at a quenching temperature of 900 to 1080 ° C. for 30 minutes, and then quenched. Residual austenite can be contained up to 30% by tempering at ° C.

  By adopting the configuration of the means of the present invention, it is possible to prevent early peeling due to a change in the white structure of the steel material in a rolling part made of rolling part steel used for an electrical accessory of an automobile engine, etc. The present invention has an excellent effect that has not been achieved so far, such as a longer life.

  The best mode for carrying out the present invention will be described. First, test steels A to H in Table 1 are steels for rolling parts of the present invention, and I to O are comparative steels. A steel having the component elements shown in Table 1 was melted to produce a 100 kg steel ingot. Next, the steel ingot was formed into a round bar of φ65 mm by forging and spheroidizing annealing was performed by a conventional method. That is, the spheroidizing annealing was performed at a maximum temperature of 800 ° C., and spheroidizing annealing was performed for a total of 14 hours.

  In the above comparative steel, the comparative steel I has C lower than the lower limit of the present invention. Comparative steel J has less Cr than the lower limit of the present invention. Comparative steel K has more Cr than the upper limit of the present invention. In the comparative steel L, N is less than the lower limit of the present invention, and Ti is greater than the upper limit of the present invention. Comparative steel M has less Cr than the lower limit of the present invention. Comparative steel N has more N than the upper limit of this invention. In the comparative steel O, Cr is less than the lower limit of the present invention, and N is less than the lower limit of the present invention.

  Thrust-type rolling fatigue test specimens were collected from the spheroidized specimen steel and comparative steel, respectively. Further, these collected specimens were quenched. As shown in Table 2, quenching was carried out at a quenching temperature of 840 to 1100 ° C. for 30 minutes depending on the Cr content, and then quenched in oil at 50 ° C. Next, tempering was performed at 180 ° C. for 90 minutes as (a). After measuring the hardness after quenching and tempering and the amount of retained austenite, tempering was further performed at 300 ° C. for 90 minutes as (b), and the stability of retained austenite was evaluated. The amount of retained austenite was calculated from the integrated intensity of X-ray diffraction lines from the austenite phase and the martensite phase.

Next, a rolling fatigue test was performed in order to know the effect of improving the fatigue strength and life of the steel for rolling parts. This rolling fatigue test was performed using a thrust type rolling fatigue tester at a surface pressure of 5290 MPa. In this case, the test piece was charged with hydrogen before the test to reproduce the peeling caused by the white texture change. The conditions for this hydrogen charge are immersed in a 5% NH ₄ SCN solution for 48 hours, and after the immersion, the surface is lapped to remove corrosion products, and a thrust type rolling fatigue test is started after 1 hour of hydrogen charge. It was.

Table 2 shows the test results of these thrust type rolling fatigue test pieces together with the amount of retained austenite. Furthermore, Figure 1 shows the thrust-type rolling fatigue test of L ₁₀ life and austenite amount of decrease in relation comparative steels and rolling part steel of the present invention as a graph.

As can be seen in Table 2, in the test steels A to H of the present invention, the amount of retained austenite shown in% after quenching and tempering is the lowest (8) of the test steel H after tempering at 180 ° C. 9% and (b) 8.2% after tempering at 300 ° C., the highest being (a) 28.2% after tempering at 180 ° C. and (b) 27. 6%. Therefore, all of the test steels A to H are 8 to 30% of the object of the present invention and satisfy the scope of the present invention. Further, (a)-(b) of the amount of decrease in retained austenite showing the stability of retained austenite during use (which does not decompose into martensite) is the largest, 2 of the test steel C. 4%. That is, each of the test steels A to H has a decrease in retained austenite (a)-(b) of 3% or less, which satisfies the scope of the present invention. As a result, the L ₁₀ life of the rolling fatigue test showing the life of the materials of the test steels A to H is 15.5 × 10 ⁶ cycles or more of the test steel G as seen in FIG.

To the above-described invention steel I of comparative steels, comparative steels K, comparative steel N austenitic reduction is to satisfy 3% or less, L ₁₀ life of the thrust-type rolling fatigue test 4.0 × 10 ⁶ cycles It is inferior to the following and the test steel of the present invention. Further comparative steels J, comparative steel L, comparative steels M, Comparative Steel O austenitic reduction is greater than 3%, addition, L ₁₀ life of the thrust-type rolling fatigue test and 4.4 × 10 ⁶ cycles or less It is inferior to the test steel of the present invention.

  Furthermore, the test steel of A ′ has the same composition as that of the test steel of A, but in order to change the amount of retained austenite after tempering, after quenching from 1080 ° C., first, subzero treatment of −80 ° C. × 60 min. It is what went. Next, after tempering at 180 ° C. for 90 minutes in (a) and measuring the quenching, the hardness after tempering and the amount of retained austenite, tempering at 300 ° C. for 90 minutes in (b) was further performed. It is shown as a comparative steel for confirming the effect of the sub-zero treatment.

This subzero-treated A ′ test steel has a residual austenite amount of 6.8% after 180 ° C. tempering in (a), and a residual austenite amount after 300 ° C. tempering in (b) of 6.5%. Although the amount of decrease in retained austenite is as low as 0.3% and is stable by subzero treatment, the amount of retained austenite is less than 8%, which is inferior to the present invention. Moreover, the L ₁₀ life of the rolling fatigue test was 5.5 × 10 ⁶ and the life was short, and the effect of the subzero treatment was not obtained.

  Furthermore, the test steel of A ″ has the same composition as the test steel of A, but was subjected to a quenching treatment from 1100 ° C. in order to change the amount of retained austenite after tempering. After tempering at 180 ° C. for 90 minutes and measuring the quenching, hardness after tempering, and the amount of retained austenite, tempering at 300 ° C. for 90 minutes in (b) was further performed.

In the test steel of A ″, the amount of retained austenite after tempering at 180 ° C. in (a) is 40.2%, and the amount of retained austenite after tempering at 300 ° C. in (b) is 38.9%. although reduction of the residual austenite is less stable and 1.3%, an excessive amount of retained austenite exceeds 30%, therefore the hardness is lowered, L ₁₀ life of the rolling fatigue test was 6.2 The life is shortened to × 10 ⁶ .

Graph showing the relationship between L ₁₀ life and austenite amount of decrease in the thrust-type rolling fatigue test of Comparative Steel and rolling part steel of the present invention.

Claims (4)

In mass%, C: 0.50 to 1.1%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Cr: 3.0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015% When the remaining austenite amount is 8 to 30% after quenching from a quenching temperature of 900 to 1080 ° C. and tempering at 150 to 230 ° C., and then kept at 300 ° C. A steel for rolling parts, characterized in that a reduction in retained austenite is 3% or less. In mass%, C: 0.50 to 1.1%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Cr: 3.0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015% Further, steel containing one or more selected from Ni ≦ 2.0%, Mo ≦ 1.5%, V ≦ 1.0%, Nb ≦ 0.3%, and the balance being Fe and inevitable impurities The amount of retained austenite after quenching from a quenching temperature of 900 to 1080 ° C. and tempering at 150 to 230 ° C. is 8 to 30%, and when the temperature is kept at 300 ° C. thereafter, the amount of decrease in retained austenite is 3% or less. It is a steel for rolling parts. In mass%, C: 0.50 to 1.1%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Cr: 3.0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015% The residual austenite is 8-30% after quenching from a quenching temperature of 900-1080 ° C. and tempering at 150-230 ° C., and then remaining at 300 ° C. A rolling part characterized in that the reduction of austenite is 3% or less. In mass%, C: 0.50 to 1.1%, Si: 0.1 to 1.2%, Mn: 0.2 to 1.5%, Cr: 3.0 to 10.0%, N: 0.02 to 0.05%, P ≦ 0.03%, S ≦ 0.01%, Al: 0.005 to 0.05%, Ti ≦ 0.005%, O ≦ 0.0015% Further, steel containing one or more selected from Ni ≦ 2.0%, Mo ≦ 1.5%, V ≦ 1.0%, Nb ≦ 0.3%, and the balance being Fe and inevitable impurities The amount of retained austenite after quenching from a quenching temperature of 900 to 1080 ° C. and tempering at 150 to 230 ° C. is 8 to 30%, and the decrease in retained austenite when kept at 300 ° C. is 3% or less. Rolling parts characterized by being.

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