JPH062898B2 - Short-time spheroidizing heat treatment method for high carbon chromium bearing steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a method for spheroidizing heat treatment of high carbon chromium bearing steel containing no molybdenum, which is specified in JIS-G4805.

[Conventional technology]

Among the bearings, rolling bearings, for example, are subjected to severe rolling loads for a long time during use. On the other hand, ring-shaped members such as the inner ring and the outer ring of the rolling bearing are manufactured by cutting a seamless pipe and then quenching it.

When manufacturing the inner ring and outer ring of this rolling bearing using the high carbon chrome bearing steel specified in JIS-G4805, a seamless pipe made of high carbon chrome bearing steel is used for the cutting and abrasion resistance before cutting. It is customary to perform a spheroidizing heat treatment in order to secure a rolling life to give a microstructure in which spheroidized carbide is uniformly and finely distributed.

The high carbon chromium bearing steel specified in JIS-G4805 is SUJ1 to SUJ3 containing no molybdenum.
And SUJ4 and SUJ5 containing molybdenum, but SUJ1 to SUJ3 containing no molybdenum.
Also contains molybdenum in an amount of 0.08% by weight or less.

As the spheroidizing heat treatment method for molybdenum-free high carbon chromium bearing steel, the following two methods are generally used.

Hold at 770-800 ℃ for about 2 hours, then 600 ℃
To 10 ° C./hr or less.

Hold at 780 to 820 ° C. for 2 to 3 hours, hold at 720 ° C. for 2 to 3 hours, and then 650 at a speed of about 5 ° C./hr.
Slowly cool to ℃.

For high carbon chromium bearing steel containing molybdenum, the following method is disclosed in Japanese Examined Patent Publication No. 57-36966.

After Ac held about 1 hour at a temperature of ₁ b point to Ac ₁ point e, repeated Ar ₁ b point to Ar ₁ between the point e 10 ° C. / hr or less at a rate of annealing annealed more than 4 times.

[Problems to be Solved by the Invention]

However, both methods require extremely slow gradual cooling and thus require an extremely long processing time.

That is, in the method (2), a wide temperature range of about 200 ° C. is gradually cooled at a rate of 10 ° C./hr or less, and in the method (70), a temperature range of 70 ° C. is gradually cooled at an extremely slow rate of about 5 ° C./hr. Therefore, when the heating time and the heating and holding time are combined, each method requires a processing time of about 20 hours or more. Also in the method (1), although the slow cooling temperature range is relatively narrow (about 50 ° C.), four or more repetitions are required, so the processing time is at least about 25 hours.

In the heat treatment that the treatment time exceeds 20 hours,
Since batch processing is required and the processing time is long, the processing efficiency is significantly reduced.

In view of such a situation, the present invention provides a spheroidizing heat treatment method for a high carbon chromium bearing steel that enables a significant reduction in treatment time by combining a plurality of heat treatments in which cooling from heating and holding is rapidly cooled. is there.

[Means for Solving the Problems]

According to the method of the present invention, as shown in FIG. 1, for a high carbon chromium bearing steel containing less than 0.08 wt% of Mo in impurities,
After heating and holding at a temperature of 780 ° C. or higher and 820 ° C. or lower, Ar ₁
The primary spherical treatment is performed by cooling at a rate of 50 ° C./hr or more and 200 ° C./hr or less to a temperature less than point b, and then A
After heating to a temperature of Ac ₁ b point + 40 ° C. or less, exceeding c ₁ b point, to a temperature of Ar ₁ b point or less, 50 ° C./hr or more 200
The secondary spheroidizing treatment of cooling at a rate of ° C / hr or less is repeated three times or more.

However, Ar ₁ b point: A ₁ transformation point starting temperature during cooling Ac ₁ b point: A ₁ transformation point starting temperature during heating [Action] According to the method of the present invention, at the processing stage (hot working) of the material. The generated lamellar pearlite decomposes in the primary spheroidizing treatment, and carbide precipitates during the cooling process. In this first spheroidizing treatment method, since the cooling is carried out rapidly as slow cooling, the carbide is not uniformly dispersed and the size thereof is not constant after cooling. Therefore, the secondary spheroidizing process is subsequently performed.

When the secondary spheroidizing treatment is carried out, the carbide is redissolved by the heating and holding, and the large carbide is slightly reduced, but the small carbide disappears. Then, in the cooling process, the residual carbide further grows. Since the cooling at this time is also rapid cooling as slow cooling, new fine carbide is generated in the cooling process, but this disappears in the second secondary spheroidizing treatment.

In this way, by repeating solid solution and precipitation of carbide, fine carbide finally disappears, and only residual carbide grows to almost the same size, and a perfectly spheroidized structure is obtained.

Although the method of the present invention requires heating and cooling a plurality of times, any cooling is rapid cooling as gradual cooling, so that the overall processing time is significantly shortened as compared with the conventional method.

Although the method among the conventional methods performs heating and cooling a plurality of times, the aim is to secure a spheroidized structure when molybdenum is contained, and the idea of shortening the processing time for those that do not contain molybdenum is I don't have it. Therefore, the cooling temperature range and the cooling rate are different from those of the method of the present invention.

In the method of the present invention, the heating and holding temperature in the primary spheroidizing treatment was 780 ° C. or higher and 820 ° C. or lower,
If the temperature is lower than 80 ° C, the lamellar pearlite is insufficiently decomposed and the decomposition time is extended, and if the temperature is higher than 820 ° C, the carbide excessively forms a solid solution, and a new nucleus is partially formed in addition to the growth of residual carbide in the subsequent cooling process. This is because there is a risk of being generated and causing pearlite transformation.

The heating rate here does not matter. When the material to be treated is a steel pipe, a heating rate at which the temperature difference in the thickness direction is not significant is preferable. Further, the holding time may be as long as the decomposition of the lamellar perlite is guaranteed, and is usually 30 minutes or more.
However, holding for a long time lowers the efficiency, so in practice, about 1 hour is preferable.

The cooling in the primary spheroidizing was to less than Ar 1 _b point, because there is no growth of globular carbides in Ar 1 _b point or higher.

The cooling temperature here is set to 200 ° C./hr or less, but if the temperature exceeds 200 ° C./hr, the efficiency can be improved, but a large amount of carbide is newly deposited and is densely deposited, resulting in a significantly incomplete spheroidized structure. This is because a uniform spheroidized structure cannot be obtained even if the secondary spheroidizing treatment is repeated. On the contrary, when cooling is slow, there is no problem in terms of the structure, but the efficiency decreases, so the lower limit is 50 ° C./hr or more, preferably 100 ° C./hr or more.

As described above, the primary spheroidizing treatment gives an incomplete spheroidized structure. In order to obtain a perfect spheroidized structure, the secondary spheroidizing treatment is subsequently performed three times or more.

The heating temperature in the secondary spheroidizing treatment is set to exceed the Ac ₁ b point, A
The reason for setting c ₁ b point + 40 ° C. or lower is that the Ac ₁ b point + 40 ° C. does not re-dissolve inhomogeneous fine carbides that have undergone the primary spheroidization treatment at the Ac ₁ b point or lower. This is because such heating is not only unnecessary for re-solidification, but also prolongs the heating time and the cooling time after heating.

The cooling in the secondary spheroidization to Ar ₁ b point 200
The reason why the process is performed at a rate of not more than C / hr is for the same reason as in the case of cooling in the primary spheroidizing treatment.

Performing the secondary spheroidizing treatment three times or more means that although grain growth of carbide occurs in the secondary spheroidizing treatment once or twice, it does not reach a complete spherical structure and is repeated three times. This is because a perfect spheroidized structure can be obtained and the grain growth is saturated only after the treatment (1). Since the grain growth is practically saturated after 3 times, the treatment of 4 times or more exclusively prolongs the treatment time.
Therefore, it is preferably 5 times or less, more preferably 4 times or less.

The method of the present invention is intended for high carbon chromium bearing steels specified by JIS-G4805 that do not contain molybdenum, specifically SUJ1 to SUJ3. When the method of the present invention is applied to, for example, SUJ2 among them, the conventional method
A good spheroidized structure which was required for about 0 hours can be obtained by the treatment for about 10 hours.

〔Example〕

 Next, the results of the confirmation test of the effect of the present invention will be described.

For SUJ2 type seamless steel pipes (73φ × 7t) as hot-finished whose composition is shown in Table 1, heated to 760 to 840 ° C. as a primary spheroidizing treatment and held for 1 hour,
It is cooled to 700 to 760 ° C at a rate of 100 to 300 ° C / hr, and subsequently 760 ° C x 3 as a secondary spheroidizing treatment.
After heating for 0 minutes, the annealing of cooling to 700 ° C. at a rate of 100 ° C./hr was repeated 1 to 5 times. Sample steel Ar
_{The 1} b point is 720 ° C., and the Ac ₁ b point is 750 ° C. Second
For the steel pipe after the subsequent spheroidizing treatment, Rockwell hardness was investigated in order to evaluate the degree of spheroidizing. Table 2 shows the survey results and the required processing time, together with the processing conditions.

As is clear from Table 2, according to the present invention, a treatment for less than 10 hours can obtain a good spheroidized structure comparable to the conventional treatment for 20 hours, and the treatment time can be greatly reduced to 1/2 or less. It will be possible.

In FIG. 2, only the number of times of the secondary spheroidizing treatment was changed.
No. 2 (3 times), 6 (4 times), 7 (5 times), 14 (2
10 shows the relationship between the number of treatments and the Rockwell hardness and the particle size of the spherical structure for 15 times (1 time) and 16 times (0 times). The grain size of the spheroidized texture was investigated by scanning electron microscopy.

As is clear from FIG. 2, the particle diameter of 0.5 μm was almost reached by the second spheroidizing treatment three times, and the Rockwell hardness was 8 μm.
It drops to about 9. This effect is almost saturated after three treatments.

FIG. 3 is a micrograph showing the effects of the cooling rate in the primary spheroidizing treatment and the number of times of the secondary spheroidizing treatment on the spheroidizing, as compared with the case of the conventional example. Take a photo N
Samples were extracted between the treatments for o.2, 3, 9, and 17.

As is clear from FIG. 3, the cooling rate is 200 ° C./hr.
If the following is satisfied, a good spheroidized structure equivalent to or better than the conventional method can be obtained by performing the secondary spheroidizing treatment three times or more.

〔The invention's effect〕

As is clear from the above description, the method of the present invention treats a high carbon chromium bearing steel for about 10 hours or less, and achieves good spheroidization comparable to or exceeding the conventional treatment of 20 hours or more. Grant tissue. As described above, the method of the present invention significantly shortens the treatment time while maintaining a good structure, and the continuous reduction of treatment time enables continuous treatment. It is something that can be improved.

[Brief description of drawings]

FIG. 1 is a heat pattern diagram of the method of the present invention, and FIG. 2 is a chart showing the influence of the number of times of the secondary spheroidizing treatment on spheroidizing with respect to Rockwell hardness and the grain size of the spheroidizing structure. FIG. 3 is a photomicrograph of a metallographic structure showing the effects of the cooling rate in the primary spheroidizing treatment and the number of times of the secondary spheroidizing treatment on the spheroidizing, regarding the spheroidizing structure shape.

Claims (1)

[Claims] 1. A high carbon chromium bearing steel containing less than 0.08% by weight of Mo in impurities, after being heated and held at a temperature of 780 ° C. or higher and 820 ° C. or lower, then heated to a temperature of less than Ar ₁ b point at 50 ° C. /
The primary spheroidizing treatment of cooling at a rate of not less than hr and not more than 200 ° C./hr is performed, and subsequently Ac ₁ point is exceeded and Ac ₁ b point +
After heating to a temperature of 40 ° C or lower, 5 up to a temperature of Ac ₁ b or lower.
A short-time spheroidizing heat treatment method for high carbon chromium bearing steel, characterized in that the secondary spheroidizing treatment of cooling at a rate of 0 ° C / hr or more and 200 ° C / hr or less is repeated three times or more.