JPH11229083A - Bearing steel with excellent machinability - Google Patents

Abstract

(57) [Problem] To provide a bearing steel having excellent machinability in an annealed state and having tool life, cutting surface roughness, and cutting property equal to or more than conventional spheroidized annealed steel. . SOLUTION: In weight%, C: 0.6 to 1.1%, Si:
0.1-0.7%, Mn: 0.2-1.6%, Cr:
The ferrite phase contains 0.8 to 1.6%, the balance being Fe and unavoidable impurities, and has a two-phase structure including a lamellar pearlite phase having an area ratio of 30 to 60% in a ferrite phase in which spherical cementite is present. It is a bearing steel with excellent machinability, characterized by spheroidized cementite + lamellar pearlite as seen in the micrograph of FIG.
The machinability shows properties equivalent to or better than those of the conventional spheroidal annealing material in terms of tool life, cutting surface roughness, and chip disposability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to bearing steel having excellent machinability in an annealed state.

2. Description of the Related Art JIS is a typical example of bearing steel.
G4805 SUJ2. When processing SUJ2 into a bearing component, for example, a bearing race, a process of starting production of a steel material from hot or warm forging and a process of directly starting cutting are common. In the process that starts with direct cutting, the microstructure and hardness of the material delivered from the steel maker to the processing maker greatly affect the machinability. For example, since SUJ2 is a hypereutectoid steel, the structure after hot rolling becomes a cementite + lamellar pearlite structure, and the hardness is extremely hard. When cutting or drilling with a lathe, the life of the cutting tool is extended. It is short and practically impossible to cut.

[0002] As a method of improving the machinability of a steel material, it is conceivable to first add a free-cutting element such as Pb, S or Ca. P
b is present as a low-melting metal inclusion, which makes it easier to break chips by the molten metal embrittlement effect, and furthermore, shows a lubricating effect at the interface between the chip and the tool surface, thereby reducing cutting resistance and extending tool life. . MnS acts as a stress concentration source and exhibits a notch effect, and also improves chip controllability and tool life. In addition, Ca forms oxides or sulfides to cover the tool surface, and has an effect of preventing the tool from being worn. However, if these free-cutting substances are present in parts that are used in high-strength areas such as bearing steel and require rolling fatigue life, those parts will act as defects, reducing the life of the parts. There is a problem of doing. Therefore, SUJ2
When cutting bearing steel represented by the formula (1), it is necessary to improve machinability by controlling the structure before cutting and the hardness. Therefore, in general, prior to cutting, spheroidizing annealing treatment is performed to spheroidize cementite, the structure is made into ferrite + spheroidized cementite, and the hardness is significantly reduced before cutting. It is.

However, this spheroidizing annealing requires a sufficient time at an appropriate temperature and is not economical. Also, aggregates of fine cementite that are present without being completely spheroidized cementite are called sorbic pearlite, which may cause a roughened material surface after cutting or a reduction in tool life. Furthermore, SUJ2 subjected to spheroidizing annealing has a problem that it is deformed due to low hardness when chucking in lathe processing or when punching the inner diameter of a bearing raceway cold. Further, there is also a problem that handling scratches are likely to be caused due to low material hardness.

[0004] As a method of improving machinability depending on the structure, JP-A-7-54100 and JP-A-9-8780.
There is one. These are intended to improve machinability by coexisting soft ferrite and hard bainite structure. However, in the case of bearing steel such as SUJ2, it is difficult to form such a structure itself, and the same machinability improvement effect is not necessarily obtained because the weight percent level of C is significantly different. Absent.

[0005] Therefore, by optimizing the heat treatment of the bearing steel, the cost of the heat treatment is reduced, and the development of a bearing steel having an appropriate hardness and a machinability equal to or higher than that of the conventional spheroidizing annealing is developed. Is desired. Of course, it is premised that the quenching and tempering treatment ultimately provides a rolling fatigue life that is important as bearing characteristics.

[0006]

When the time for spheroidizing annealing of SUJ2 is shortened in order to reduce the heat treatment cost, the above-mentioned sorbic pearlite appears, and the machinability such as cutting surface roughness and tool life is increased. Deteriorates.

In the case of SUJ2, it is said that the larger the spheroidized cementite particle size is, the smaller the number is, the lower the hardness is, and the higher the machinability is. Therefore, in order to obtain an appropriate hardness without deteriorating the machinability, it is necessary to use a structure other than ferrite and spheroidized cementite.

[0008] An object of the present invention is to solve the above-mentioned conventional problems by optimizing the structure and hardness, and to provide a bearing steel excellent in machinability.

[0009]

Means for solving the above problems of the present invention are as follows. In the invention of claim 1, C: 0.6-1.1%, Si: 0.1-% by weight.
0.7%, Mn: 0.2 to 1.6%, with the balance being F
e, a bearing having excellent machinability, comprising a two-phase structure containing a lamellar pearlite phase having an area ratio of 30 to 60% in a ferrite phase in which spheroidized cementite is present and in which spheroidized cementite is present. For steel.

According to the second aspect of the present invention, C: 0.
6 to 1.1%, Si: 0.1 to 0.7%, Mn: 0.2
~ 1.6%, Cr: 0.8 ~ 1.6%, Mo: 0.15 ~ 0.7%, the balance is Fe and unavoidable impurities, and spheroidized cementite is present This is a bearing steel excellent in machinability, characterized by having a two-phase structure including a lamellar pearlite phase having an area ratio of 30 to 60% in a ferrite phase.

According to the third aspect of the present invention, the annealing hardness is 9
3. A bearing steel excellent in machinability according to the means of claim 1 or claim 2 having a hardness of 2 to 97 HRB.

The inventors of the present invention have conducted tests on various machinability by changing the annealing structure and hardness of the bearing steel in various ways. As a result, in the bearing steel represented by SUJ2, the structure of the cutting material was made spherical. When a two-phase structure in which a lamellar pearlite phase having an area ratio of 30 to 60% (hereinafter, “%” represents weight%) is present in a ferrite phase in which cementite is present is used, a conventional ferrite + spheroidized cementite structure is obtained. Tool life and cutting surface roughness equal to or greater than
It shows that it shows good chip controllability. The annealing time to obtain this structure is shorter than the conventional spheroidizing annealing time, and the hardness is also harder than the conventional ferrite + spheroidized cementite structure, so deformation during chucking on a lathe or cold punching. The problem and the handling flaw problem hardly occur.

The reasons for adding the alloy components and the reasons for limiting the composition range in the present invention are described below. C is required to have a C content of 0.6% or more in order to maintain quenching hardness of 60 HRC or more required for a bearing material. On the other hand, if it exceeds 1.1%, it is difficult to prevent coarse precipitation of proeutectoid cementite in the cooling process after hot rolling, so that C is 0.6 to 1.1%.
Limited to the range.

[0014] Si is an element effective in deoxidizing steel and is also a very effective element for improving the rolling fatigue life. However, if the content of Si is less than 0.1%, the above-mentioned effect is exhibited. If it is not possible to exceed 0.7%, the matrix hardness increases and the machinability deteriorates. Therefore, Si is set to 0.1 to 0.7%.

Mn is used as a deoxidizing / desulfurizing agent.
In addition, the element has a large effect of enhancing hardenability and is a very effective element for improving the rolling fatigue life. In order to exhibit these effects effectively, the content must be 0.2% or more. However, if added in excess of 1,6%, the lamellar pearlite area ratio increases and adversely affects machinability, so Mn was set to 0.2 to 1.6%.

Cr not only improves the hardenability, but also
It is an element that improves the spheroidization of cementite and is an indispensable element for producing the braid of the steel of the present invention. 0.8% or more is required to take advantage of the effect. Also C
r has almost no effect of strengthening the ferrite and is not harmful even if it is increased, but the upper limit is set to 1.6% in consideration of economy.
Therefore, Cr is set to 0.8 to 1.6%.

Mo is added when necessary in order to improve the hardenability, but in order to exert its effect effectively,
0.15% or more, and 0.7
% Is not only economically disadvantageous but also deteriorates the machinability.
%.

Next, the reasons for limiting the annealing hardness and the annealing structure in the present invention will be described. In the present invention, it is an important point that the lamellar pearlite phase is present in an area ratio of 30 to 60% in the ferrite phase containing spheroidized cementite. In the case of such a two-phase structure, the tool life, the cutting surface roughness, and the cutting surface are increased, although the hardness of the material is increased as compared with the conventional ferrite + spheroidized cementite structure material. Machinability such as processability is equivalent or higher. The conventional spheroidized annealed structure aims to reduce the area ratio of hard cementite by spheroidization and to improve the machinability only by reducing the hardness of the bearing steel. On the other hand, the steel of the present invention actively uses cementite in order to improve machinability. In other words, by using two types of cementite, spheroidized cementite and lamellar pearlite which exists in an appropriate ratio, the lamella pearlite has a role of a stress concentration source in a shearing region at the time of cutting. This is because the lamellar pearlite phase is harder than the ferrite phase containing spheroidized cementite, and in this case, the deformed portion is concentrated in the ferrite phase containing spheroidized cementite, so that voids and cracks are generated and cracks are generated. This is presumably because the propagation of susceptibility tends to occur at the interface between the ferrite phase containing spheroidized cementite and the lamellar pearlite phase, resulting in embrittlement of the material and improvement in machinability. This effect is small when the lamellar pearlite area ratio is less than 30%, and when it exceeds 60%, the machinability deteriorates. Therefore, the area ratio of the lamellar pearlite phase in the ferrite phase containing spheroidized cementite was limited to the range of 30 to 60%.

Further, the hardness is set to 92 to 97 HRB in addition to the fact that the material hardness in the case of the above-described two-phase structure falls within this range, as well as chucking with a lathe and cold punching of the bearing raceway ring inner diameter. In order to prevent deformation in the process and to reduce handling flaws as compared with the conventional spheroidal annealing material, a hardness of 92 HRB or more is required, and the hardness is 97 HR.
If B is exceeded, the machinability deteriorates.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (1) In order to carry out the first invention of the present invention, C: 0.6 to 1.1% and Si: 0.
1-0.7%, Mn: 0.2-1.6%, Cr: 0.8
~ 1.6% bearing steel is melted in an electric furnace, slab-rolled and hot-rolled after continuous casting, and the temperature is raised to a maximum temperature of 760 ° C for 6 hours and annealed at 760 ° C for 1 hour.
The heat treatment is performed for a total of 14 hours, with the temperature maintained at r and the gradual cooling time from 760 ° C to 700 ° C for 7 hours. The machinability of the material obtained in this way is equivalent to or better than that of the conventional spheroidal annealing material in terms of tool life, cutting surface roughness, and chip disposability.

(2) In order to carry out the second aspect of the present invention, the weight ratio of C: 0.6 to 1.1% and Si: 0.1 to
0.7%, Mn: 0.2 to 1.6%, Cr: 0.8 to
Bearing steel containing 1.6%, Mo: 0.15 to 0.7% is melted in an electric furnace, and then subjected to continuous casting, followed by slab rolling and hot rolling, and as annealing, up to a maximum temperature of 760 ° C. Heating time of 6 hours, holding at 760 ° C. for 1 hour, 700 ° C. to 700
The heat treatment is performed for a total of 14 hours with a gradual cooling time of 7 hours to ℃.
The machinability of the material obtained in this way is equivalent to or better than that of the conventional spheroidal annealing material in terms of tool life, cutting surface roughness, and chip disposability.

(3) In order to carry out the third invention of the present invention, the weight ratio of C: 0.6 to 1.1% and Si: 0.1 to
0.7%, Mn: 0.2 to 1.6%, Cr: 0.8 to
1.6%, and if necessary Mo: 0.15-0.
Steel for bearings containing 7% is melted in an electric furnace, continually cast, subjected to slab rolling and hot rolling, and is annealed for 6 hours at a maximum temperature of 760 ° C and 1 hour at 760 ° C.
The heat treatment is performed for a total of 14 hours with a holding time of 7 hours from 760 ° C to 700 ° C. The hardness of the material thus obtained is 92-97 HRB, and the machinability is tool life,
In terms of cutting surface roughness and chip disposability, it exhibits properties equivalent to or better than conventional spheroidizing annealing.

[0023]

Embodiments of the present invention will be described below.

[0024]

[Table 1] A steel containing the components shown in Table 1 and consisting of the remaining Fe and unavoidable impurities was melted in an electric furnace, and after continuous casting, slab rolling and hot rolling were performed to obtain a φ65 mm rod. As shown in FIG. 1, (b) Heat treatment No. 2,
(C) Heat treatment No. 3, (d) Heat treatment No. 4 were subjected to heat treatment of four types of patterns. For these, various machinability tests (carbide tool turning chip disposal, carbide tool turning tool life, carbide tool turning surface roughness, turning high-speed tool life, high-speed drilling tool life) were performed. For some materials, thrust rolling fatigue life test (Pmax 540)
kg / mm2).

Table 2 shows the microstructure and hardness measurement results after annealing, Table 3 shows the machinability test conditions, and Table 4 shows the test results. FIG. 1 is a microphotograph of steel treated with heat treatment pattern 1, showing spheroidized cementite + lamellar pearlite, and FIG. 1 is heat treatment pattern 2
The microstructure photograph of the steel treated with the above shows spheroidized cementite.

[0026]

[Table 2]

[0027]

[Table 3]

[Table 4]

[0028]

As described above, the steel of the present invention
Since it has a two-phase structure of ferrite phase and lamellar pearlite phase in which spheroidized carbide is dispersed, shortening of heat treatment time and cost reduction can be achieved, but hardness is higher than conventional spheroidized structure, but lamellar pearlite is Since the shearing part at the time of cutting concentrates on the ferrite as a source of stress concentration, the generation and propagation of cracks at the time of shear deformation and chip generation becomes easier than the conventional spheroidized structure, and as a result, machining in the annealed state It is a bearing steel that has excellent properties and exhibits properties equivalent to or better than conventional spheroidizing annealing in tool life, cutting surface roughness, and chip disposability.

[Brief description of the drawings]

FIG. 1 is a diagram showing heat treatment patterns of the present invention and a comparative example.

FIG. 2 is a micrograph of the steel of the present invention.

FIG. 3 is a micrograph of a comparative steel.

Claims (3)

[Claims] 1. C .: 0.6 to 1.1% by weight, S
i: 0.1 to 0.7%, Mn: 0.2 to 1.6%, C
r: a two-phase structure containing 0.8 to 1.6%, the balance being Fe and unavoidable impurities, and including a lamellar pearlite phase having an area ratio of 30 to 60% in a ferrite phase in which spheroidized cementite is present. Bearing steel excellent in machinability characterized by being constituted. 2. C: 0.6-1.1% by weight, S
i: 0.1 to 0.7%, Mn: 0.2 to 1.6%, C
r: 0.8 to 1.6%, and Mo: 0.1
The ferrite phase contains 5 to 0.7%, the balance being Fe and inevitable impurities, and has a two-phase structure including a lamellar pearlite phase in an area ratio of 30 to 60% in a ferrite phase in which spheroidized cementite is present. Bearing steel with excellent machinability characterized by the following: 3. The bearing steel having excellent machinability according to claim 1, wherein the steel has an annealing hardness of 92 to 9.
Bearing steel with excellent machinability characterized by 7HRB.