JPH04103715A - Method for spheroidizing high-carbon chromium bearing steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of spheroidizing annealing high carbon chromium bearing steel specified by SUJ2 in JIS-G4805.

[Conventional technology]

In recent years, the inner and outer rings of rolling bearings have been manufactured by cutting and quenching seamless steel pipes. When using high carbon chromium bearing steel specified by SUJ2 in JIS-G4805 as the material, the seamless steel pipe made of high carbon chromium bearing steel has machinability and
In order to ensure wear resistance, rolling life, etc., it is customary to perform spheroidizing annealing to give a microstructure in which spheroidized carbides are uniformly and finely distributed.

Conventional heat treatment methods commonly used as spheroidizing annealing for steel types such as high carbon chromium bearing steel specified by SUJ2 in JIS-04805 all involve extreme heat treatment of 10°C/Hr or less after long-term temperature maintenance. Requires slow slow cooling. Moreover, this slow cooling may be repeated, and the heat treatment time exceeds 20 hours, significantly deteriorating the thermoeconomic efficiency.

Therefore, the present applicant has developed a method for high carbon chromium bearing steel, as shown in Figure 4, after heating and holding it at a temperature exceeding 780°C and below 820°C, the applicant heated it at 20'C to the desired temperature of A r 1b.
Following the primary spheroidization treatment in which the material is cooled at a rate of less than / Hr, it is heated to a temperature of 40 °C or less in terms of Ac1b across the Ac1b point, and then heated to a temperature of 200 °C / Hr or less to a temperature of A r 1b or less.
We proposed a rapid spheroidizing annealing method in which the secondary spheroidizing process, which is cooled at a rate of less than Hr, is repeated three or more times (Japanese Patent Application Laid-Open No. 123-123).
Publication No. 4519).

[Problem to be solved by the invention]

In this rapid spheroidizing annealing method proposed by the present applicant, the secondary spheroidizing process is performed three or more times after the first spheroidizing process, so the number of heating and cooling times is as small as four or more times (though not many). Since each cooling is performed at a high speed of 200"C/Hr or less, the processing time is halved to about 10 hours. Moreover, the obtained &I
The texture is a uniform, fine, spheroidized structure comparable to the conventional 20-hour method of grilling sardines. However, the secondary spheroidization treatment performed three or more times requires heating each time, and the energy cost is still high.

An object of the present invention is to provide a spheroidizing annealing method that requires fewer heating times and a shorter processing time than the above method or the fast spheroidal annealing.

[Means for solving the problem] As mentioned above, the disadvantage of the rapid spheroidizing annealing proposed earlier by the applicant is that the number of secondary spheroidizing treatments is relatively large, three or more times. be. The present inventors have investigated various measures to reduce the number of repetitions of the secondary spheroidization treatment in this rapid spheroidization annealing and to suppress the increase in particle size of the spheroidized carbide caused by this. As a result, as shown in Figure 1, the cooling rate in the second secondary spheroidization process was increased to 20
By slightly lowering the temperature from below 0°C/Hr to below 75°C/Hr, a uniform and fine spherical shape comparable to that obtained by repeating two primary spheroidization processes and three or more secondary spheroidization processes can be obtained. It was found that carbides can be obtained.

The present invention was made based on the above findings, and is based on the JIS-G
For high carbon chromium bearing steel specified by SUJ2 in 4805, the first step is to heat and hold at a temperature of 780°C or more and 820'C or less, and then cool it at a rate of 200°C/Hr or less to a temperature of A r 1b or less. Following the spheroidization process, as a second spheroidization process, the A c 1b point is moved and the A c 1b point +
After heating to a temperature of 40°C or less, cool it at a rate of 200°C/Hr or less to a temperature of A r 1b or less, and then
After heating to a temperature exceeding the Ac1b point +40℃ or less, A
The gist of this invention is a spheroidizing annealing method for high carbon chromium bearing steel, which is characterized by cooling to a temperature below the r 1b point at a rate of 75° C./Hr or below.

[For production]

According to the spheroidizing annealing method of the present invention, lamellar pearlite formed during the processing stage (hot working) of the material is decomposed in the first spheroidizing treatment, and carbide precipitation occurs in the cooling process.

However, since this cooling is rapid, the carbides are not uniformly dispersed and their size is not constant after cooling. Therefore, a second spheroidization process is subsequently performed.

When the secondary spheroidization treatment is performed, the carbides are solid-dissolved again by heating and holding, and although the large carbides become slightly smaller, the small carbides disappear, and in the cooling process, the remaining carbides grow further. However, if this cooling is rapid, new fine carbides are generated during the cooling process.

The two-speed spheroidizing annealing method previously proposed by the present applicant involves repeating the secondary spheroidizing process with rapid cooling three or more times to sequentially eliminate fine carbides produced by rapid cooling and remove relatively large carbides. grow into a completely spherical tissue. On the other hand, in the spheroidizing annealing method of the present invention, the cooling rate in the second secondary spheroidizing treatment is relatively slow at 75°C/Hr or less to suppress the precipitation of fine carbides, and to suppress the precipitation of fine carbides. Growth is promoted, thereby obtaining a spheroidized tissue comparable to the case where the secondary spheroidization process is performed three or more times with two secondary spheroidization processes.

The spheroidizing annealing method of the present invention requires fewer times of primary spheroidizing treatment than the rapid spheroidizing annealing method previously proposed by the applicant, and can reduce heating costs.

Although the cooling rate in the second secondary spheroidization process is slow,
An extremely slow speed of 10° C./Hr or less is not necessary, and the processing time can be shortened by reducing the number of processing times.

In the spheroidizing annealing method of the present invention, the heating holding temperature in the first spheroidizing treatment is set to 780°C or higher and 820°C or lower because if it is lower than 780°C, the decomposition of the lamellar pearlite will be insufficient and the decomposition time will also be extended. , 820'C or more, there is a risk that carbides will form an excessive solid solution, and in the subsequent cooling process, in addition to growing residual carbides, new nuclei will be partially generated, resulting in pearlite transformation.

Here, the heating rate does not matter, but if the material to be treated is a steel pipe, a heating rate that does not cause a noticeable temperature difference in the thickness direction is preferred. Further, the holding time is sufficient as long as decomposition of the lamellar pearlite is ensured, and is usually 30 minutes or more.

However, holding for a long time reduces processing efficiency, so in practice it is preferably about 1 hour.

The reason why the cooling temperature in the primary spheroidization treatment was raised or lowered by the A r 1b point is that spheroidal carbides do not grow at temperatures exceeding the A r 1b point.

The reason why the cooling rate was set to 200'C/Hr or less is that although efficiency can be improved at 200°C/Hr, a large amount of carbide newly precipitates and precipitates in a dense manner, resulting in extremely incomplete spherical shape. This is because a uniform spheroidized structure cannot be obtained even if the secondary spheroidization process is repeated. On the other hand, if the cooling is slow, the efficiency will decrease, but there will be no problem with the structure. Therefore, the lower limit is not particularly defined.

In terms of efficiency, it is best to cool as quickly as possible, preferably 5
It is 0°C/Hr or more, more preferably 100°C/Hr or more.

As described above, a spheroidized structure is obtained by the first spheroidization process, although it is incomplete. In order to obtain a completely spheroidized structure, two secondary spheroidization treatments are subsequently performed.

The heating temperature in the second spheroidization treatment is A, c1b point sardine,
The reason why the A c + b point is set to be below +40°C is that below the Ac1b point, the fine non-uniform carbide produced in the primary spheroidization treatment does not become solid solution again, but heating that exceeds the Ac1b point +40°C This is because not only is it unnecessary for re-dissolution, but it also prolongs the heating time and the cooling time after heating.

The cooling rate here is 200'C/Hr or less in the first secondary spheroidization treatment. This is due to the same reason as the cooling in the primary spheroidization process.

The cooling rate in the second secondary spheroidization treatment was 75°C.
/Hr or less. This means that above 75℃/Hr,
Fine carbides precipitate during the cooling process, and the growth of the remaining relatively large carbides becomes insufficient, resulting in large grain sizes (because a uniform and good spheroidal structure cannot be obtained. Point of spheroidization) Therefore, the slower the cooling rate, the better, but an extreme decrease in the rate will result in a significant extension of the processing time.Actually, a cooling rate of 25° C./Hr or more is desirable.

(Example) Examples of the present invention will be described below in comparison with conventional examples and comparative examples.

A seamless steel pipe made of JIS-G4805SUJ2 steel whose composition is shown in Table 1 was made into a seamless steel pipe as hot finished.
Spheroidizing annealing was carried out according to the treatment patterns (1) to (2) shown in (C), respectively, and the average grain size of the carbide after annealing was investigated using a scanning electron microscope.

The A r 1b point of this steel is 720℃, and the Ac1b point is 75
It is 0. The steel pipe dimensions were an outer diameter of 75ss and a wall thickness of 7mm. Processing pattern 1 (FIG. 2(a)) is a comparative example in which the secondary spheroidizing process is performed once after the primary spheroidizing process. Same ■
(Fig. 2(b)) shows the example of the present invention in which the secondary spheroidization treatment was performed twice and the cooling rate was 25 to 75°C/HrO.
-200° C./Hr is a comparative example, and 6.2 (FIG. 2 (C)) is a conventional example submitted by the present applicant in which the secondary spheroidization treatment was performed three or more times.

Table 2 shows the details of the treatment conditions, the treatment time, and the investigation results of the carbide average particle size.

Table 1 (wt%) The example of the present invention is different from the conventional example in which the secondary spheroidization process is performed three or more times, although the second spheroidization process following the first spheroidization process is performed twice. A comparable spheroidal structure was obtained. Also,
The processing time is more than one hour shorter than the conventional example.

Figure 3 shows the conventional method in which the secondary spheroidization process is performed three or more times.
It shows the change in the heat treatment unit when the operation pattern was switched to the method of the present invention in which the secondary spheroidization treatment was performed twice.

By implementing the method of the present invention, the number of times of heating in the secondary spheroidization treatment was reduced, and the energy cost was reduced in ms.

〔Effect of the invention〕

As is clear from the above explanation, the method of spheroidizing high carbon chromium bearing steel of the present invention is the rapid spheroidizing annealing proposed by the present applicant, which significantly shortens the processing time compared to the conventional general annealing method of this type. The processing time is even shorter than that of the method, and
The number of times of heating is small, making it possible to significantly save energy costs.

[Brief explanation of the drawing]

The first panel is a graph schematically showing the heat pattern of the method of the present invention. The 4th is a graph showing the implementation effect in terms of energy cost reduction effect, and the 4th is a graph schematically showing the heat pattern of the conventional method.

Claims (1)

[Claims] (1) For high carbon chromium bearing steel specified by SUJ2 in JIS-G4805, after heating and holding at a temperature of 780°C or higher and 820°C or lower, 200°C to a temperature of Ar_1b point or lower.
Following a primary spheroidization process of cooling at a rate of /Hr or less,
As the second spheroidization process, Ac_1 beyond the Ac_1b point
After heating to a temperature below point b +40°C, cool at a rate below 200°C/Hr to a temperature below point Ar_1b, and then
A spheroidizing annealing method for high carbon chromium bearing steel, which comprises heating to a temperature exceeding the Ac_1b point and below the Ac_1b point +40°C, and then cooling to a temperature below the Ar_1b point at a rate of 75°C/Hr or below.