JP2007204803A - Steel parts for bearing, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel parts for a bearing, which are subjected to quenching treatment suitable for inner rings, outer rings and balls of the bearing and the like, and have improved fatigue characteristics. <P>SOLUTION: A base steel material for the steel parts has a composition comprising 0.6-1.5 mass% C, 0.1-1.0 mass% Si, 0.1-1.5 mass% Mn, 0.1 mass% or less Al, 0.05-2.0 mass% Cr, and the balance Fe with unavoidable impurities. The manufacturing method comprises the steps of: spheroidizing carbides in the base steel material; then working it at a working rate of 20% or more; and subsequently quenching it so that the former austenite grains in the quenched surface layer shall have a diameter of 3.5 μm or smaller. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a bearing component such as a bearing inner and outer ring, a bearing ball, and the like, and a manufacturing method thereof, and more particularly, a bearing steel in which fatigue properties are improved by refining a prior austenite grain size of a quenched surface layer portion. Regarding parts.

Steel components for bearings such as bearings used in automobiles and machines are required to have excellent rolling fatigue characteristics. Steel parts for bearings are used after being quenched and tempered at sites where fatigue characteristics are required.
As a method for improving the rolling fatigue life, for example, Patent Document 1 discloses that an S53C level hypoeutectoid steel (ferrite, pearlite structure) is subjected to high-frequency heat treatment to an austenite single-phase region at least twice to obtain a prior austenite. A method to refine the grain size and improve the fatigue life is described. The achieved prior austenite grain size is 6.2 μm even with the smallest grain size, and the rolling fatigue life is 1.2 to 1.5 times that of conventional materials. Degree.
On the other hand, although hypereutectoid steel is not described in Patent Document 1, generally, in the case of hypereutectoid steel, the prior austenite grain size is about 6 to 10 μm even with a normal quenching material. Therefore, it is said that the rolling fatigue life cannot be improved unless the prior austenite grain size is refined by further optimum heat treatment.
JP 2002-256336 A

  The present invention has been developed in view of the above-described present situation, and an object of the present invention is to provide a steel part for a bearing that has an improved rolling fatigue life and a manufacturing method thereof.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have performed this processing in a material that has been subjected to processing with a processing rate of 20% or more after spheroidizing annealing and then quenched. It has been found that the rolling fatigue life is improved as compared with a material having a processing rate of less than 20% when not applied or when applied. And the improvement effect of the rolling fatigue life by the above-mentioned working rate of 20% or more was found to be particularly prominent when the prior austenite grain size of the quenched portion is 3.5 μm or less.

The present invention has been completed by further studying the obtained knowledge, and the gist of the present invention is as follows.
(I) C: 0.6 to 1.5 mass%,
Si: 0.1-1.0mass%,
Mn: 0.1-1.5mass%,
Al: 0.1 mass% or less and
Cr: 0.05-2.0mass%
A steel part for bearings using a steel material having a composition comprising the balance Fe and unavoidable impurities, and after subjecting the steel material to a spheroidizing treatment of carbide, the processing rate is 20% or more A steel part for bearings obtained by processing and then quenching, wherein the prior austenite grain size of the quenched surface layer after quenching is 3.5 μm or less.

(Ii) the composition further comprises
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Mo: 1.0mass% or less,
W: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
Sb: 0.0050 mass% or less and
N: Steel component for bearing as described in (i) containing 1 type or 2 types or more chosen from 0.01 mass% or less.

(Iii) The steel part for bearing according to (i) or (ii), wherein the hardness of the quenched surface layer part is Hv 700 or more.

(Iv) C: 0.6 to 1.5 mass%,
Si: 0.1-1.0mass%,
Mn: 0.1-1.5mass%,
Al: 0.1 mass% or less and
Cr: 0.05-2.0mass%
, Spheroidizing carbide to the steel material having the balance Fe and unavoidable impurities, and then processing with a processing rate of 20% or more, then, Ac3 point -10 ° C ~ Ac3 point For bearings where the average heating rate between temperatures is 0.5 ° C / s or higher, and the quenching process is performed at a temperature of Ac3 point or higher and Ac3 point + 130 ° C or lower and holding time of Ac3 point or higher is 500 seconds or shorter. Manufacturing method of steel parts.

(V) the composition further comprises
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Mo: 1.0mass% or less,
W: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
Sb: 0.0050 mass% or less and
N: The manufacturing method of the steel part for bearings as described in (iv) containing 1 type, or 2 or more types chosen from 0.01 mass% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the steel component for bearings excellent in rolling fatigue life can be obtained easily, and it is very useful industrially.

The present invention will be specifically described below.
The steel parts for bearings of the present invention are processed into the shape of steel parts for bearings, such as inner and outer rings of bearings, bearing balls, bearing rollers and needles, through a forming process (forging, cutting, etc.), from steel materials, preferably steel bars or wires. After that, it is manufactured by quenching. In order to obtain the steel part for bearing of the present invention, it is necessary to optimize the composition of the steel material, the material structure, the processing rate before quenching, the quenching conditions, and the structure after quenching.

[Steel composition]
The component composition of the steel material will be described.

C: 0.6-1.5mass%
C is an element necessary for securing the hardness necessary for obtaining the fatigue life of the part in the quenched portion. If it is less than 0.6 mass%, sufficient hardness and fatigue strength cannot be obtained in the quenched portion. On the other hand, when the content exceeds 1.5 mass%, the workability before quenching (shearability, forgeability) is deteriorated. Therefore, the C content is in the range of 0.6 to 1.5 mass%.

Si: 0.1-1.0mass%
Since Si is an element that improves the rolling fatigue life, it is necessary to contain 0.1 mass% or more. However, when it contains exceeding 1.0 mass%, like C, the workability before quenching (shearability, forgeability) is deteriorated. Therefore, the Si content is 0.1 to 1.0 mass%.

Mn: 0.1-1.5mass%
Since Mn is an element that improves hardenability, it must be contained at 0.1 mass% or more. However, when it contains excessively, the workability (shearability, forgeability) before hardening will deteriorate. For this reason, the upper limit of the content is set to 1.5 mass%.

Cr: 0.05-2.0mass%
Cr is contained in an amount of 0.05 mass% or more because it has the effect of improving the hardenability and reducing the hardness before quenching by promoting the spheroidization of carbide and improving the workability. However, even if added in excess of 2.0 mass%, the effect is saturated, so the range is 0.05 to 2.0 mass%.

Al: 0.1 mass% or less
Al is preferably contained because it is an element having a strong deoxidizing action and an effect of cleaning steel. However, when it is added in excess of 0.10 mass%, the cleaning effect of the steel is rather lowered and the fatigue life is lowered, so the content is made 0.1 mass% or less. More preferably, it is 0.005-0.1 mass%.
The above is the basic component composition of the present invention, but the following elements may further be contained.

S: 0.03 mass% or less
As described later, S is also present in steel as an unavoidable impurity, but S combines with Mn to form MnS and has the effect of improving the machinability of steel, so it is positive with 0.03 mass% as the upper limit. You may add to. However, if added over 0.03 mass%, MnS becomes the starting point of cracking and significantly reduces the fatigue life. Therefore, when added, the upper limit of the content is set to 0.03 mass%.

Cu: 1.0 mass% or less
Cu may be added because it has the effect of increasing the hardness of the quenched portion by improving the hardenability. When adding in order to acquire this effect, the upper limit shall be 1.0 mass%.

Ni: 1.0 mass% or less
Ni has the effect of increasing the hardenability of the steel and improving the toughness of the hardened part. Therefore, you may add 1.0 mass% as an upper limit. In particular, when Cu is added, in order to suppress hot brittleness, it is particularly preferable to add Ni to the amount of Cu added.

Mo: 1.0 mass% or less
Mo may be added because it has an effect of improving hardenability and an effect of improving temper softening resistance. However, if it exceeds 1.0 mass%, workability deteriorates. Is 1.0 mass% or less.

W: 1.0mass% or less
W may be added because it has an effect of improving hardenability. However, if it is added in an amount exceeding 1.0 mass%, the workability deteriorates. Therefore, when W is added, the content is made 1.0 mass% or less.

Nb: 0.5 mass% or less
Nb may be added because it has an austenite grain growth-inhibiting action due to nitride formation. However, if it exceeds 0.01 mass%, the fatigue characteristics deteriorate. Therefore, when Nb is added, the Nb content should be 0.01 mass% or less.

B: 0.01 mass% or less
B may be added because it has the effect of improving hardenability, but the effect is saturated even if added over 0.01 mass%, so when added, the upper limit is 0.01 mass%.

Sb: 0.0050 mass% or less
Sb has the effect of delaying the microstructure change and has the effect of preventing the deterioration of rolling fatigue characteristics, so it may be added. However, if the content exceeds 0.0050 mass%, the toughness deteriorates, so when added, the content is made 0.0050 mass% or less.

N: 0.01 mass% or less
N is present in steel as an inevitable impurity, as will be described later, but it forms nitrides (or carbonitrides) and is useful for refining γ grains, so it is actively added up to 0.01 mass%. May be. Since excessive addition exceeding 0.01 mass% deteriorates the workability of steel, when adding, 0.01 mass% is made an upper limit.

  The balance other than the elements described above is Fe and inevitable impurities. Inevitable impurities include P and O, and the above-described S and N are mixed as inevitable impurities even when not actively added. P is acceptable up to 0.05 mass%, and O is acceptable up to 0.0150 mass%. Further, even if S is not actively added, 0.01 mass% is the upper limit, and N is contained up to 0.0080% even if not actively added.

[Processing rate before quenching]
The processing rate before quenching forms the basis of the present invention, and as described above, it is important to perform processing with a processing rate of 20% or more. Processing may be performed in multiple steps, and the total processing rate may be 20% or more.

  The processing rate mentioned here means a reduction in area of the cross-sectional area in the case of a rod / wire, and means a reduction rate in the case of a plate. Furthermore, in the case of complicated shapes such as casting, if the Vickers hardness is improved by 50 points or more than the material, the processing rate is considered 20% or more.

  Fig. 1 shows the reduction in area and life ratio (B10 life / reduction in each area reduction) for steel with the average prior austenite grain size of the surface hardened part at 3.3 μm and 6.1 μm in two levels. B10 life at a rate of 0%). Here, 1.0 mass% C-0.2 mass% Si-0.35 mass% Mn-1.5 mass% Cr steel was used as the steel material. Then, after the spheroidizing treatment, drawing was performed at various processing rates (area reduction rate) in the range of 0 to 58%, and the influence of the area reduction rate was investigated. In order to clarify the improvement of the life due to the reduction in area during processing, the lifespan is increased by increasing the reduction in area based on the B10 life of the 0% area reduction sample in each old austenite grain size steel. The increase was determined by B10 life at each area reduction ratio / B10 life at 0% area reduction ratio (hereinafter referred to as life ratio).

  As shown in Figure 1, the life ratio of the sample with an average prior austenite grain size of 6.1μm is saturated after improving to 1.3 times with a reduction in area of about 10%, whereas the sample with an average prior austenite grain size of 3.3μm However, when the area reduction ratio is less than 20%, the life ratio tends to improve as the area reduction ratio increases, and when the area reduction ratio is 20%, the life ratio reaches about 2.3 times and becomes saturated.

[Old austenite grain size of hardened surface layer]
Steel parts for bearings are usually used after being quenched and tempered at sites where rolling fatigue characteristics are required. In the present invention, a hardened portion (hereinafter referred to as a hardened surface layer portion) is provided in a surface layer portion particularly requiring rolling fatigue characteristics, and the prior austenite grain size of the hardened surface layer portion is 3.5 μm or less. The hardened surface layer portion means a surface layer portion that has been subjected to quenching and has a martensite structure, but not only the surface layer but also a structure having a martensite structure for the entire thickness, The prior austenite grain size should just be 3.5 micrometers or less.

  In addition, it is preferable that the thickness of the surface layer part whose average prior austenite particle size is 3.5 μm or less is at least 0.2 mm or more. According to the investigations by the inventors, as described above, the rolling fatigue life is remarkably improved when the raw material before quenching is processed with a processing rate of 20% or more. Has been found to be a case of 3.5 μm or less. In order to make this easier to understand, FIG. 2 shows the relationship between the prior austenite grain size and the reduction in area during processing performed between the spheroidizing treatment and the quenching treatment of the material. It can be seen that when the prior austenite particle size is 3.5 μm or less, the life ratio is improved by a factor of two or more compared to when the prior austenite particle size is more than 3.5 μm.

Here, the hardness of the hardened surface layer portion is preferably 700 or more in terms of Vickers hardness (hereinafter referred to as Hv). If it is less than Hv700, the hardness is insufficient and the rolling fatigue life of the bearing steel part tends to decrease. In order to set the hardness of the hardened surface layer portion to Hv700 or higher, it is necessary to consider the heating temperature during the tempering process, but details of the tempering process will be described later.
The structure of the steel part for bearing according to the present invention has been described above. Next, the method for manufacturing the steel part for bearing according to the present invention will be described.

[Steel structure before quenching]
The structure of the steel before quenching needs to have a carbide spheroidized by spheroidizing treatment, but the structure other than the spherical carbide is a single phase structure of ferrite, bainite, martensite or two or more of these. It may be a multiphase structure. However, as will be described later, in the present invention, a combination of ferrite and spheroidized carbide is preferable from the viewpoint of workability in consideration of processing with a processing rate of 20% or more before quenching. In the present invention, the spheroidized carbide refers to a carbide having an aspect ratio of 3.0 or less.

  In addition, although it does not prescribe | regulate especially as a method of spheroidizing annealing, what is necessary is just to perform by the normal heat processing pattern optimal for a composition. For example, 1.0 mass% -0.2Si-0.4Mn-1.5Cr steel may be held at about 780 ° C. for 5 hours and gradually cooled.

[Hardening conditions]
As described above, in the present invention, the average prior austenite grain size of the quenched surface layer portion needs to be 3.5 μm or less. For this purpose, at least the surface layer that is formed after the spheroidizing treatment and the processing rate of 20% or more is performed. The heating conditions during the quenching treatment of the part were set to an average heating rate of 0.5 ° C / s or higher between temperatures from Ac3 point -10 ° C to Ac3 point, heating temperature from Ac3 point to Ac3 point + 130 ° C, and further to Ac3 The retention time above the point needs to be 500 seconds or less.

  Here, the Ac3 point means a temperature at which the reverse transformation from ferrite, bainite or martensite to austenite is completed during heating. If the heating temperature is less than the Ac3 point, the reverse transformation to austenite is not completed, so that a complete martensite structure cannot be obtained after quenching, and sufficient hardness required for rolling fatigue characteristics cannot be obtained. On the contrary, when the heating temperature is higher than Ac3 point + 130 ° C., the austenite grain growth is rapidly promoted, so that the prior austenite grain size of the structure after quenching exceeds 3.5 μm.

About the heating rate at the time of the heating at the time of a quenching process, it is necessary to set it as 0.5 degree-C / s or more on the average between the temperature of Ac3 point-10 degreeC-Ac3 point. If the heating rate is slower than 0.5 ° C / s in this temperature range, the austenite grain size will not be refined due to a decrease in nucleation driving force to austenite, and the prior austenite grain size of the quenched structure exceeds 3.5 μm. End up.
Furthermore, when the holding time of the Ac3 point or more exceeds 500 seconds, it becomes a sufficient time for grain growth, and the prior austenite grain size of the structure after quenching exceeds 3.5 μm.

  The quenching process under the above conditions may be performed only once or multiple times. In the case of performing quenching a plurality of times, this heating rate condition may be applied only at the time of the final quenching process (only the Nth time when performing the N times quenching process). In the quenching process that is performed prior to the final quenching process (in the case of N quenching process, the first to N-1 quenching processes), the structure after quenching is a bainite or martensite structure (single-phase or rephased) As long as it is composed of residual spherical carbides, and heat treatment limited to the final quenching step is not particularly required. However, if heating is performed at such a high temperature that the residual spherical carbide dissolves, the effect of suppressing the austenite grain growth caused by the spherical carbide during the final quenching disappears, and the grains become coarse, and the carbon to the parent phase Therefore, it is necessary to make the Acm point (the temperature at which spherical carbide dissolves in austenite and becomes an austenite single phase) at the time of quenching before the final quenching. is there.

When performing multiple quenching treatments, a quenching surface layer having fine prior austenite grains can be obtained by applying the above quenching conditions for all quenching treatments. It doesn't matter. However, the number of quenching treatments is preferably two from the viewpoint of industrial productivity and prevention of cost increase.
A quenching surface layer part having an average prior austenite grain size of 3.5 μm or less is obtained by performing the quenching treatment under the conditions described above.

[Tempering]
In the present invention, a tempering process may be performed after the quenching process. However, when performing tempering treatment, if the tempering temperature is too high, the quenched surface layer portion is softened, the fatigue strength is reduced, and the effect of refining the prior austenite grain size of the quenched surface layer portion is reduced. When tempering is performed, the temperature is preferably 200 ° C. or lower. When the tempering temperature is 200 ° C. or less, a hardened surface layer portion having a hardness of Hv 700 or more can be obtained by using the steel having the above-described composition.

  After the quenching and tempering treatments are performed under the above-described conditions, surface treatment such as shot peening and finishing surface polishing treatment are performed as necessary to obtain steel parts for bearings.

  A 100 kg steel ingot having various compositions shown in Table 1 was soaked at 1250 ° C. for 15 hours, and then hot forged at 850 ° C. or more to obtain a 13-20 mmφ bar steel. The steel bar is subjected to spheroidizing annealing (SA) to form a steel structure composed of ferrite and carbides having an average aspect ratio shown in Tables 2 and 3, and then various surface reduction rates shown in Tables 2 and 3 are used. Cold drawing was performed to 13.0mmΦ. In order to produce a radial type rolling fatigue test piece from the center of this wire, first, it was roughly processed into a cylindrical test piece having a diameter of 12.2 mmΦ, and after quenching under the heat treatment conditions shown in Tables 2 and 3, Tempering was performed at 170 ° C. or 250 ° C., and a cylindrical test piece having a length of 12 mmΦ × 22 mm was finished.

About the obtained test piece, the rolling fatigue life to peeling was investigated using the radial type fatigue testing machine (Cylinder type fatigue testing machine made by NTN), and the rolling fatigue characteristic was evaluated. The fatigue test was performed on 20 samples at a Hertz stress of 5884 MPa (600 kgf / mm ² ) and a rotational speed of 46400 cpm, and a 10% failure probability life (hereinafter referred to as B10 life) was obtained. For each steel type, the area reduction rate at the time of cold drawing is 0, that is, the value obtained by dividing the B10 life of each test piece by the B10 life of the test piece prepared without performing cold drawing. As a ratio, the degree of improvement in rolling fatigue life was evaluated.

Also, in order to investigate the Vickers hardness of the surface layer of the test piece and the prior austenite grain size of the surface layer, the untested cylindrical test piece is cut so that the circumference of the test piece can be observed by corresponding to the transfer surface of the fatigue test. And embedded in resin and polished. At this cut surface (hereinafter referred to as C section), Vickers hardness within 0.1 mm from the surface was measured at a load of 2.94 N (300 gf) at 90 ° intervals and averaged. The average prior austenite grain size of the surface layer is corroded using the former austenite grain boundary appearing liquid (manufactured by JFE Steel, Gamma R), and 4 fields of view at 90 ° intervals immediately below the C cross-section surface layer using SEM 2 each Images were taken at 5,000 magnifications and then quantified using the cutting method. The number of cross-sections of the average prior austenite grain size obtained by the cutting method, which are divided into four in the vertical and horizontal directions in each SEM image, and this line segment (108 μm per field of view) intersects the prior austenite grain boundaries (X) Was calculated as old austenite particle size (μm) = 108 / (0.89 × X), and then obtained by averaging the values obtained from the 8 SEM images.
Tables 2 and 3 also show the Vickers hardness, average prior austenite grain size, and fatigue characteristics of the quenched surface layer portion.

  As can be seen from Tables 2 and 3, when the average prior austenite grain size of the quenching surface layer is 3.5 μm or less, the fatigue life is improved by making the area reduction rate in the drawing process before quenching 20% or more. You can see that On the other hand, during the quenching process, the average heating rate between Ac3 point -10 ° C and Ac3 point is less than 0.5 ° C / s, or in the steel whose average prior austenite grain size exceeds 3.5 µm, Although the fatigue life can be improved by reducing the area reduction ratio in the drawing process to 20% or more, the degree of improvement is small.

It is a graph which shows the influence which the area reduction rate at the time of the process before hardening has on a rolling fatigue life. It is a graph which shows the influence which the average prior austenite particle size of a hardening surface layer part has on a rolling fatigue life.

Claims (5)

C: 0.6-1.5 mass%,
Si: 0.1-1.0mass%,
Mn: 0.1-1.5mass%,
Al: 0.1 mass% or less and
Cr: 0.05-2.0mass%
A steel part for bearings using a steel material having a composition comprising the balance Fe and unavoidable impurities, and after subjecting the steel material to a spheroidizing treatment of carbide, the processing rate is 20% or more A steel part for bearings obtained by processing and then quenching, wherein the prior austenite grain size of the quenched surface layer after quenching is 3.5 μm or less. The composition further comprises
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Mo: 1.0mass% or less,
W: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
Sb: 0.0050 mass% or less and
N: The steel part for bearings of Claim 1 containing 1 type, or 2 or more types chosen from 0.01 mass% or less.   The steel part for bearings according to claim 1 or 2 whose hardness of said hardened surface layer part is Hv700 or more. C: 0.6-1.5 mass%,
Si: 0.1-1.0mass%,
Mn: 0.1-1.5mass%,
Al: 0.1 mass% or less and
Cr: 0.05-2.0mass%
, Spheroidizing carbide to the steel material having the balance Fe and unavoidable impurities, and then processing with a processing rate of 20% or more, then, Ac3 point -10 ° C ~ Ac3 point For bearings where the average heating rate between temperatures is 0.5 ° C / s or higher, and the quenching process is performed at a temperature of Ac3 point or higher and Ac3 point + 130 ° C or lower and holding time of Ac3 point or higher is 500 seconds or shorter. Manufacturing method of steel parts. The composition further comprises
S: 0.03 mass% or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Mo: 1.0mass% or less,
W: 1.0 mass% or less,
Ti: 0.01 mass% or less,
Nb: 0.5 mass% or less,
B: 0.01 mass% or less,
Sb: 0.0050 mass% or less and
N: The manufacturing method of the steel component for bearings of Claim 4 containing 1 type, or 2 or more types chosen from 0.01 mass% or less.

Images