JPH0673437A - Method for uniformizing carbide grain in spheroidizing annealing of high carbon chromium bearing steel wire rod - Google Patents

Abstract

PURPOSE:To shorten working time for a bearing steel, to reduce the unevenness of the working time, to reduce the difference between the temp. history at the inner part and that at the outer part of a wire rod coil by preventing the coarsening and the unevenness of carbide in the spheroidizing annealing of a high carbon chromium bearing steel wire rod. CONSTITUTION:In the spheroidizing annealing by heating the wire rod coil of the high carbon chromium bearing steel to a prescribed temp. higher than transformation temp. (AC1) and slowly cooling after soaking, a heat treatment furnace prepared so as to be possible to use for heating by convection and provided with a stirring fan 18 is used and the heating operation is executed while flowing from the inner part to the outer part of the wire rod coil 20 by forced convection of the atmosphere in the furnace. Further, the carbide in made fine and uniformly distributed so that the heating velocity in the temp. range of the transformation temp. (AC1)+ or -50 deg.C becomes >=70 deg.C/hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for sizing a carbide in which a carbide such as cementite in a pearlite structure is formed into a fine spherical shape in a spheroidizing annealing of a high carbon chromium bearing steel wire to form a uniform structure.

[0002]

BACKGROUND ART As a bearing steel used as a bearing material or the like, SUJ specified in JIS-G-4805 is used.
High carbon chromium bearing steels such as 1 and SUJ2 are well known. Further, conventionally, in such a high carbon content steel, prior to quenching, a heat treatment called spheroidizing annealing is performed, and by adjusting the carbide into a spherical shape, the toughness is improved, strain is suppressed, and uniform It is designed to obtain various tissues and hardness. And, the size and uniformity of the spheroidized carbide have a great influence on the abrasivity and machinability when processing a bearing ball or the like from bearing steel, so regarding the sizing of the carbide in the spheroidizing annealing, Quality that is stricter than before is required.

By the way, for such spheroidizing annealing, various methods such as a slow cooling method, a constant temperature holding method, a repeating method and a cold working heat treatment method have been conventionally known. Among them, the slow cooling method is
The heat treatment is performed according to the temperature curve as shown in FIG. That is, first, the temperature is raised from the point a at a heating rate of 20 ° C. to 60 ° C./hour, and after heating to a predetermined temperature higher than the transformation point (A _c1 ) of the bearing steel, for example, to a point b around 800 ° C. Hold for a few hours and soak. Then, after that, the temperature is lowered from the point b ′ at a cooling rate of 20 ° C. to 60 ° C./hour so that the point c near the transformation point (A _c1 ) is slowly cooled particularly slowly to reach the point d. Of.

Corresponding to the temperature curve from point a to point d, the carbide in the pearlite structure of the bearing steel changes as shown in FIGS. 2 (a) to 2 (d). That is,
FIG. 2A corresponding to the point a shows cementite which is a typical carbide.
At the point b after the heating operation, the state is divided into the state of FIG. Next, during the soaking operation, a part of the carbide is solid-dissolved in the austenite, while an appropriate amount remains, and at the b'point, the result shown in FIG.
The state becomes (b '). Then, during the process of transformation of austenite into ferrite during the cooling operation, the remaining carbides serve as nuclei to promote agglomeration, and at point c, as shown in FIG.
Then, by slowly cooling the vicinity of the transformation point particularly slowly, at the point d, the carbide grows into a spherical shape as shown in FIG. 2 (d).

However, in such a conventional heat treatment method, coarse spherical carbides are partially formed,
At the stage of quenching after spheroidizing annealing, the average grain size of the carbide increases and the grain size of the carbide becomes σ:
There has been a problem that there is a large variation of about 0.28 μm to 0.35 μm. And, therefore, the abrasiveness and machinability of the bearing steel deteriorate, and the processing time becomes longer,
There was an inherent problem that the processing time varied. Therefore, various temperature curves for spheroidizing annealing through heating operation, soaking operation, and cooling operation have been conventionally studied in order to refine and uniformize carbides of bearing steel, but still satisfactory results. Has not been obtained.

Further, since the bearing steel is usually subjected to spheroidizing annealing in the state of the wire rod coil, it is unavoidable that the heating conditions are different in each part of the wire rod coil due to the mass effect of the wire rod coil. The outside of the wire rod coil satisfying the heating condition, more specifically, the outer peripheral portion and the axial end portions of the wire rod coil can follow the temperature curve of FIG. 1 well, while the wire rod coil with the poor heating condition is present. In the inside, more specifically, in the inner peripheral portion and the central portion in the axial direction of the wire coil, the heat conduction becomes poor, and the temperature history becomes as shown by the broken line in FIG. As a result, coarsening of carbides easily occurs inside the wire coil, and that portion may not be able to meet the quality requirements.

[0007]

The present invention has been made against the background of the above circumstances, and the problem to be solved is to reexamine the heat treatment conditions in spheroidizing annealing of a high carbon chromium bearing steel wire, It is intended to make the carbide fine and uniform so that the carbide can be satisfactorily sized. Also,
The purpose is to reduce the difference in temperature history between the parts of the wire rod coil so that the carbide can be satisfactorily sized in the entire wire rod coil.

[0008]

[Means for Solving the Problems] In order to solve the above problems, the inventors of the present invention have made various investigations, and in the heating operation of spheroidizing annealing, when the heating rate is low, especially the transformation point (A _c1 )
When gradual heating or soaking is carried out in the vicinity, the starting point of separation of the carbide is reduced, the number of carbides is reduced, and the carbides in contact with the grain boundaries are agglomerated, so that a part thereof is coarsened, and It has been found that the coarsened carbide becomes difficult to form a solid solution even if it exceeds the transformation point (A _c1 ), so that the variation in grain size becomes large.

That is, the present invention has been completed as a result of further studies based on such findings. The gist of the present invention is to wire a coil of high carbon chromium bearing steel in a heat treatment furnace. , Heated to a predetermined temperature higher than the transformation point (A _c1 ), and then held for a predetermined time,
After soaking, it is gradually cooled to spheroidize the carbides in the pearlite structure.In the spheroidizing annealing, a convection-heatable furnace is used as the heat treatment furnace, and the heating operation is performed by forcing the atmosphere in the furnace. To the outside and vice versa, and the heating rate in the temperature range of the transformation point (A _c1 ) ± 50 ° C is 70 ° C / hour or more. In addition, the present invention provides a method for sizing carbides in spheroidizing annealing of a high carbon chromium bearing steel wire rod, characterized in that the carbides are refined and uniformly distributed.

[0010]

[Operation / Effect] In short, according to the method of the present invention, the heating operation in the spheroidizing annealing is performed by rapid heating so that the heating rate in the temperature range of the transformation point (A _c1 ) ± 50 ° C is 70 ° C / hour or more. Do it. Then, by carrying out such a rapid heating operation, the phase transformation energy becomes large and the number of separation initiation points of the carbides increases, so that the carbides become fine particles, and during the soaking operation, the fine carbides form a solid solution and agglomerate. Since it becomes difficult to produce coarse carbides, it is clear that when the spheroidizing annealing is completed through the soaking operation / slow cooling operation, there are many fine carbides with small variations. It has become. Then, even after the spheroidizing annealing, quenching is performed to obtain a final product, the variation in the grain size of the carbide is σ ≦
It was effectively suppressed to about 0.27 μm, and a uniform structure was obtained. Therefore, a bearing steel having excellent abrasiveness and machinability can be obtained, the processing time can be advantageously shortened, and the variation in the processing time can be reduced.

In addition, according to the method of the present invention, heating is performed while forcibly convection the atmosphere in the furnace, so that the heating conditions tend to be poor and the heat conduction tends to be poor inside the wire coil (inner peripheral portion and axial center). The heat is effectively transferred to the department. Therefore, the difference in temperature history between the inside and outside of the wire rod coil is effectively reduced, so that the carbide particles are effectively sized for the entire wire rod coil, and the entire wire rod coil satisfies the quality requirements. is there.

[0012]

EXAMPLES Hereinafter, representative examples of the present invention will be shown to clarify the present invention in more detail. However, the present invention is not limited by the description of such examples. Needless to say, it is not something to receive. Further, in addition to the following examples, the present invention may be variously modified, modified, improved, etc. based on the knowledge of those skilled in the art without departing from the spirit of the present invention. Should be understood.

First, a coil of SUJ2 steel wire rod (diameter 5.5 mm) (1t for 1 step) was prepared and housed in a heat treatment furnace in a stacked state of 2 steps. As the heat treatment furnace, any furnace having a structure capable of convection heating can be used, but here, a furnace having a structure as shown in FIG. 3 was used. In this heat treatment furnace, a housing chamber 12 for the wire rod coil 20 is provided in a furnace body 10.
An air outlet 14 is formed on the upper wall, while an air inlet 16 is formed on the lower wall. Further, a stirring fan 18 is attached in the vicinity of the air outlet 14, and the air in the accommodation chamber 12 is caused to flow out from the air outlet 14 by the agitation fan 18 so that the space between the furnace body 10 and the accommodation chamber 12 is changed. Through the air inlet 1
It is designed to be allowed to flow in again from 6. Then, the wire coil 20 is accommodated in the accommodation chamber 1 inside the coil.
It was arranged so that the two inlets 16 were located, and a closing plate 22 was attached to the upper end of the coil. As a result, the air flowing in from the air inlet 16 passes through the gap between the wire rods of the wire rod coil 20 and is forcibly convected from the inner peripheral portion to the outer peripheral portion. The furnace atmosphere is heated by the radiant tube 24 attached to the furnace body 10.

Then, spheroidizing annealing is started in this state,
Perform heating operation at a heating rate of 80 ° C / hour for 790
After the temperature was raised to 0 ° C., the temperature was maintained for 4 hours to carry out soaking. Then, it was gradually cooled to 660 ° C. at 20 ° C./hour, and the heat treatment was completed. Further, for comparison, the same SUJ2 steel wire coil is housed in a heat treatment furnace having no convection heating structure and heated to 790 ° C. at a heating rate of 60 ° C./hour,
After that, the same processing as above was performed.

Thereafter, with respect to each of the wire rod coils of those Examples and Comparative Examples, A portion (upper coil upper portion) and B portion
The scanning electron microscope (SE) is used in each of the parts (the boundary between the upper coil and the lower coil) and C (the lower part of the lower coil).
M) 10 fields of view (2400 μm) of 5000 times photograph trace
² ) Image analysis device (manufactured by Nireco Corporation, trade name: LU
The particle size of the carbide was examined by analysis with ZEX500). Then, the results are shown together with the heating rate (measured value) at the transformation point (A _c1 ) ± 50 ° C. of each part of the wire coil,
The results are shown in Table 1 below.

Next, in the state where the wire rod coil that had been subjected to the above spheroidizing annealing was annealed after being drawn, the grain size of carbides in each part of the wire rod coil was examined in the same manner as above, and the results are summarized in Table 1 below. Showed. Further, FIG. 4 shows an observation view (1500 times) observed under a microscope for the metal structure of the portion B of the wire rod coil of the example. Then, for comparison, with respect to the metal structure of the portion B of the wire rod coil of the comparative example,
A similar observation diagram is shown in FIG.

Further, the hardened wire rod coil was shaped and polished to prepare a bearing ball having a diameter of 7.9 mm, and the polishing processing time ratio is also shown in Table 1 below.

[0018]

[Table 1]

As is clear from the results of Table 1 and FIG. 4, in the examples, after the spheroidizing annealing, the average grain size of the carbide was 0.54 μm or less, and the deviation was σ: ≦ 0.21.
Therefore, the coarsening and the variation are well prevented.
Further, the average grain size of the carbide in the B part is 0.49 μm, the deviation is small, and the heating condition is improved satisfactorily.
It can be seen that the difference in the temperature history with the C part is effectively reduced. Further, it is shown that even in the annealing after the wire drawing work, a uniform structure is obtained by suppressing the coarsening and variation of the carbide, and as a result, the bearing working time is shortened.

On the other hand, in the comparative example, after the spheroidizing annealing, the average grain size and deviation of the carbides became large, and particularly in the portion B corresponding to the inside (axial center portion) of the wire coil during heat treatment. Coarsening and variation are remarkable. Further, in the annealing after the wire drawing, the average grain size and the deviation of the carbide are considerably increased, and as a result, the bearing processing time becomes long and varies.

[Brief description of drawings]

FIG. 1 is a graph showing an example of a conventional temperature curve for spheroidizing annealing.

FIG. 2 is an explanatory view showing a state of carbide of bearing steel which is changed by the conventional spheroidizing annealing, in which (a) is a state before heating, (b) is a state after heating, and (b ′). Shows a state after soaking, (c) shows a state during cooling, and (d) shows a state after cooling.

FIG. 3 is a cross-sectional explanatory view showing a heat treatment furnace having a convection heating structure used in Examples.

FIG. 4 is an observation view (1500 times) observed under a microscope for a metal structure of a B portion of the wire coil of the example.

FIG. 5 is an observation diagram (1500 times) observed under a microscope for a metal structure of a portion B of a wire rod coil of a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Furnace body 12 Storage chamber 14 Outlet port 16 Inlet port 18 Stirring fan 20 Wire rod coil 22 Closure plate 24 Radiant tube

Claims (1)

[Claims] 1. A high carbon chromium bearing steel wire coil is placed in a heat treatment furnace, heated to a predetermined temperature higher than the transformation point (A _c1 ), and then held for a predetermined time, soaked, and then gradually heated. In spheroidizing annealing for spheroidizing carbides in the pearlite structure by cooling, a convection heatable furnace is used as the heat treatment furnace, and the heating operation is performed by forcibly convection the atmosphere in the furnace to obtain a wire rod. The heating is performed in the temperature range of the transformation point (A _c1 ) ± 50 ° C to be 70 ° C / hour or more, and the carbide is finely divided while the coil is circulated from the inside to the outside or vice versa. A method for sizing carbides in spheroidizing annealing of high carbon chromium bearing steel wire rods, characterized in that they are uniformly dispersed.