JP5075293B2 - Mold quenching method - Google Patents

Description

  The present invention relates to a mold quenching method.

The mold is required to have high hardness and high toughness, and its characteristics are greatly influenced by the quenching method.
When raising the temperature, a higher quenching temperature is selected so that the crystal grains are not coarsened in order to dissolve the alloy elements to the maximum. Further, at the time of quenching, in order to obtain high toughness, it is necessary to simultaneously refine the crystal grains, suppress carbide precipitation at the crystal grain boundaries, and prevent bainite transformation. At this time, rapid cooling is required. On the other hand, rapid cooling increases the distortion and deformation of the mold, and thus it is necessary to appropriately control the cooling rate. For this reason, various proposals have been made in the past, and a method of achieving both low strain and high toughness by adjusting the cooling conditions from the quenching temperature is mainly employed.

  For example, in Japanese Patent Application Laid-Open No. 2006-342377 (Patent Document 1), cooling following heating to a quenching temperature is set to 20 ° C./min to 5 ° C./min in a high temperature region from the quenching temperature to 600 ° C. A marquenching method has been proposed in which a low temperature region from 400 ° C. to 200 ° C. is maintained at 1 ° C./min to 15 ° C./min after being held. Thus, it is said that a mold having low distortion and high toughness can be obtained while avoiding burning cracks. The marquenching method is a process in which the cooling during quenching is kept constant at the upper part of the martensite transformation or at a slightly higher temperature to cool after quenching, in order to prevent quench cracking due to rapid cooling. is there.

  Further, the present applicant has proposed a method for quenching a mold in Japanese Patent Application Laid-Open No. 2009-074155 (Patent Document 2) focusing on both the temperature raising side condition and the cooling side condition of the quenching temperature. That is, after the quenching and heating step of heating the temperature range from the A1 transformation point to the A3 transformation point at a rate of 100 ° C./h or more, a holding step for holding a temperature range not lower than 1150 ° C. above the A3 transformation point is performed. Next, a high temperature side quenching cooling process is performed in which the temperature range from the A3 transformation point to 600 ° C. is cooled at a rate of 5 ° C./min to 20 ° C./min, and 0.5 hours in the temperature range from 500 ° C. to 400 ° C. This is a method of passing through a low temperature side quenching cooling step of cooling a temperature range of 400 ° C. to 200 ° C. at a rate of 1 ° C./min to 15 ° C./min after passing through an interruption holding step of holding for ˜5 hours.

  Japanese Patent Application Laid-Open No. 2001-152243 (Patent Document 3) and Japanese Patent Application Laid-Open No. 2002-309314 (Patent Document 4) disclose techniques related to a quenching method by the same applicant. Specifically, a steel part is rapidly cooled to a temperature just above the martensite transformation start point (Ms point), taken out of the quenching oil, and immediately above the martensite transformation start point (Ms point) by the retained heat of the steel part. This is a technology that maintains a constant temperature until the temperature reaches a nearby temperature. Thus, a material having a small internal volume, such as a part such as a gear illustrated in paragraph 0002 of cited document 3 or a sharpened portion such as a corner portion of a prism member illustrated in paragraph 0009 of cited document 4, is preferably quenched. it can.

JP 2006-342377 A JP 2009-074155 A JP 2001-152243 A JP 2002-309314 A

  The present inventors have also studied the marquenching method, but under the conditions disclosed in Patent Document 1 and Patent Document 2 described above, it was recognized that the toughness of the mold surface was insufficient and there was room for further improvement. .

  Further, when the techniques disclosed in Patent Document 3 and Patent Document 4 are applied to a mold having a larger volume than a gear or the like, it is difficult to control the cooling rate, and the surface of the mold is Ms during rapid cooling. The temperature falls below the point. At this time, there is almost no temperature drop inside the mold, and if it is taken out from the quenching oil, the surface temperature again becomes higher than the Ms point due to the retained heat inside the mold, the metal structure becomes unstable, and the mold becomes unstable. Hardness variation will occur. In the case of a heavy mold, if an attempt is made to maintain a constant temperature until it reaches a temperature just above the Ms point, the time required for the mold becomes too long, and the productivity is remarkably deteriorated.

  An object of the present invention is to provide a quenching method that can reasonably improve the toughness of the mold surface in particular.

In order to improve the toughness of the surface of the hot mold, the present inventor
1) It is necessary to apply a cooling rate much faster than the cooling rate considering the transformation region of bainite (bainite nose) in order to reliably avoid carbide precipitation at the grain boundaries.
and,
2) The adverse effects due to the temperature difference between the surface and the interior due to the application of this fast cooling rate can be reasonably eliminated by appropriately managing the cooling interruption state.
And reached the present invention.

That is, the present invention
A heating / holding step of heating the mold and holding it in a temperature range from the A3 transformation point to 1150 ° C,
After the heating and holding step, the mold is immersed in an oil bath, and the mold surface is cooled at a rate of 80 ° C./min to 250 ° C./min by oil cooling, and the mold surface temperature is 700 ° C. or less. A first cooling step for cooling to a temperature exceeding the Ms point;
After the first cooling step, the mold is lifted from the oil tank to interrupt oil cooling, and the immersion and pulling are performed again in a temperature range where the surface temperature of the mold exceeds the Ms point, An improved holding step that repeats until the temperature difference between the surface and the interior is within 200 ° C .;
A second cooling step of cooling at a rate of 1 ° C./min to 50 ° C./min until the surface temperature of the mold reaches 200 ° C. after the improved holding step;
It is the hardening method of the metal mold | die characterized by comprising.

In the present invention, it is preferable pulling number of definitive to the improvement holding step is at least 3 times.
More preferably, the second cooling step is a mold quenching method in which the mold is cooled at a rate of 1 ° C./min to 15 ° C./min until the temperature inside the mold falls from 400 ° C. to 250 ° C.
More preferably, the second cooling step is oil cooling.
More preferably, the environment in the first cooling step, the improved holding step, and the second cooling step is a non-oxidizing atmosphere.

  According to the mold quenching method of the present invention, the surface toughness of the mold can be particularly improved, so that it can be expected to improve the life of the mold.

It is the schematic of the heat pattern which showed an example of the quenching conditions of this invention. In addition, the improvement holding process is displayed as a straight line for the sake of convenience in this drawing. It is the figure which showed the temperature measurement value at the time of the metal mold | die cooling after the 1st cooling process of this invention, and a simulation result. It is a microscope picture after applying the hardening method of the metal mold | die of this invention and performing tempering.

As described above, an important feature of the present invention is that the cooling condition during quenching is optimized in the mold quenching method. The present invention will be described below.
First, the heating / holding step will be described (FIG. 1 (4)).
The temperature of the heating / holding step of the present invention is set to a temperature range not lower than 1150 ° C. above the A3 transformation point. This is because if this temperature is lower than the A3 transformation point, the solid solution of carbides and alloy elements is insufficient, the hardness is low, and the high temperature strength is also low, so heat cracks are likely to occur. Further, when the temperature exceeds 1150 ° C., the carbide pinning the crystal grains also dissolves and the crystal grains grow abnormally.
In order to suppress the occurrence of these problems and achieve the refinement of crystal grains, a temperature range from the A3 transformation point to below 1150 ° C. is required. Preferably it is the temperature range of 1010 degreeC or more and less than 1150 degreeC.

Next, the cooling process of the present invention will be described.
In the present invention, after the heating / holding step, the mold is immersed in an oil tank and cooled until the surface temperature of the mold becomes 700 ° C. or lower (temperature exceeding the Ms point) by oil cooling. A process is performed (FIG. 1 (5)).
The reason why the first cooling step is oil-cooled in the present invention is that a region in which carbide precipitates at the grain boundaries can be surely avoided. This oil cooling surely increases the toughness of the mold surface.
The reason why oil cooling is selected in the present invention is that if it is oil cooling, the cooling capacity can be suitably adjusted by adjusting the temperature of the quenching oil. For example, if water cooling with high cooling capacity is used, the cooling rate is too high, and there is a high risk that the mold surface will undergo martensitic transformation in the first cooling process. This is because there is a possibility that the cooling of the mold surface slows down and passes through the carbide precipitation region.
The cooling rate of the mold surface in the first cooling process by oil cooling is approximately 80 ° C./min to 250 ° C./min, depending on the size of the mold, and it is a conventional blast cooling or liquid spray. Compared with the quenching method of the mold, the cooling rate is much faster.
In the first cooling step, cooling until the surface temperature of the mold becomes 700 ° C. or lower is that if oil cooling is interrupted in a temperature range exceeding 700 ° C., carbides may be precipitated at the crystal grain boundaries. Because there is. In order to eliminate the possibility of precipitation, oil cooling is performed until the surface temperature of the mold exceeds the Ms point and reaches the temperature range of Ms point + 200 ° C. When the surface temperature is cooled to the Ms point or lower, the metal structure near the surface of the mold undergoes martensitic transformation, and the metal structure becomes non-uniform during the next improved holding step. Therefore, management of the surface temperature is important in the present invention.
In the present invention, the toughness of the mold surface and the vicinity thereof can be improved by the first cooling step by oil cooling described above.

Subsequently, in the present invention, an improved holding process is performed. (Fig. 1 (6) and Fig. 2)
Since a remarkably fast cooling rate is applied in the first cooling step, the temperature difference between the mold surface and the inside becomes large. In the improved holding process, this is quickly reduced to alleviate the thermal stress.
There are two methods for this improved holding process.

  The first improved holding step is a method in which the mold is lifted from the oil tank, and the oil rejection is interrupted and allowed to cool. In this method, a large amount of heat inside the mold is transferred to the mold surface, and the surface temperature rises. Thereby, the temperature difference between the mold surface and the inside is quickly reduced.

The second improved holding step (corresponding to claim 1) is a method of repeatedly immersing the mold in the oil tank and pulling it up from the oil tank. In the second improved holding step, the temperature difference between the mold surface and the inside is quickly relaxed as in the first improved holding step, and the following effects can also be obtained.
(1) The amount of heat removed from the mold can be increased, and the cooling rate inside the mold can be increased. As a result, generation of pearlite inside the mold can be prevented more reliably.
(2) The cooling rate inside the mold surface and inside the mold is increased, and it is possible to cool the inside of the mold while avoiding the carbide precipitation region.
(3) Since the cooling speed of the whole mold is fast, it is particularly suitable for quenching large molds of 200 kg or more.
(4) The time required for quenching can be shortened and productivity can be improved.
Among these, in order to obtain the effects of (1), (2) and (4) more reliably, it is preferable to shift to the second improved holding step in a temperature range of Ms point + 25 ° C. or higher. In particular, the first time may be raised before the mold surface falls below 400 ° C. This is because the mold is often a heavy object, and if the mold is excessively cooled to the vicinity of the Ms point, the temperature difference between the mold surface and the inside of the mold is widened and the mold is easily deformed.
Moreover, it is good to start the next immersion until the temperature of the mold surface rises by 200 ° C. (preferably 100 ° C.) from the surface temperature at the time of the latest pulling.

  As a result, it is possible to prevent martensite transformation on the mold surface, increase the heat removal amount, and shorten the processing time. As the number of times of immersion in the oil tank increases, the above-described effect is particularly easily obtained, and the pulling up is preferably repeated three times or more. In particular, in the case of a large mold exceeding 200 kg, it is preferable to increase the number of repetitions of dipping and pulling by increasing the initial pulling temperature (surface temperature) and decreasing the pulling temperature sequentially as described above. .

  Which one of the first improvement holding process and the second improvement holding process described above is selected is determined, for example, in consideration of the weight and processing time of the mold, and the surface area and volume of the mold. good. For example, the second improved holding process is suitable for processing a mold of 100 kg or more in a short time.

The two improved holding steps described above are performed until the temperature difference between the surface and the inside of the mold reaches 200 ° C. or less by oil cooling. This is because if the temperature difference exceeds 200 ° C. and the second cooling step is performed, there is a risk of distortion or deformation due to the thermal stress difference. In order to prevent distortion and deformation more reliably, the temperature difference between the surface of the mold and the inside is preferably within 150 ° C.
In addition, since some molds are heavy objects of about 2 tons exceeding 100 kg, if the temperature is kept constant until the temperature is just above the Ms point as in the prior art, the processing time becomes longer and the productivity is significantly reduced. Let Therefore, depending on the mode of use, for example, the second cooling step may be performed when the temperature difference between the surface of the mold and the inside becomes 50 ° C to 200 ° C (preferably 50 ° C to 150 ° C). .
By adopting this improved holding step, it is a major feature of the present invention that a constant temperature holding furnace or a salt bath, which has been required in conventional marquenches, can be eliminated.

Next, after the improved holding step, a second cooling step is performed in which the mold is cooled at a rate of 1 ° C./min to 50 ° C./min until the mold surface temperature reaches 200 ° C. (Fig. 1 (7))
The cooling rate of this second cooling step suppresses the generation of bainite of the material to be heat-treated (mold) during cooling, and also suppresses temperature unevenness due to rapid cooling, and controls toughness, quenching distortion, and cracking. This is the cooling rate necessary for this.
When the cooling rate of the surface temperature is less than 1 ° C./min, the suppression of bainite formation is insufficient and the toughness is lowered. If it exceeds 50 ° C./min, the temperature difference of the product during the martensitic transformation becomes large, and due to the temperature unevenness during cooling, distortion tends to increase and the risk of burning cracks also increases. A preferable cooling rate of the surface temperature is 10 ° C./min to 30 ° C./min.
In the second cooling step, the cooling rate at the center is also important from the viewpoint of controlling the metal structure, and the region of 400 ° C. to 250 ° C. where bainite is generated is cooled at 1 ° C./min to 15 ° C./min. good. If it is this range, the production | generation of coarse bainite can be suppressed and toughness can be improved more reliably. Preferably, it is 5 ° C./min to 15 ° C./min, and the above effect can be obtained more reliably.
In this second cooling step, it is easy to adjust the cooling rate to the above-mentioned temperature range, and in particular, the metal structure inside the heat-treated material is not likely to be a massive bainite structure, and it is easy to obtain a cooling rate, which is oil cooling Is good.

In the present invention, the environment in the first cooling step, the improved holding step, and the second cooling step described above is preferably a non-oxidizing atmosphere. Specifically, nitrogen, an inert gas, or a vacuum atmosphere (reduced pressure atmosphere) can be applied.
This is because the quenching oil burns violently to cause a disaster such as a fire because the first cooling process is a rapid cooling from the quenching temperature to the oil tank.
In addition, when the non-oxidizing atmosphere is used, oxidation and decarburization of the heat-treated material can be prevented.

Next, the conditions for heating to the above quenching temperature will be described.
In the present invention, it is more preferable to adjust the temperature raising condition in the low temperature side quenching temperature raising step as the condition for heating to the quenching temperature (FIG. 1 (1)). In addition, the low temperature side here means the temperature range below the A1 transformation point.
The condition for the low temperature side quenching temperature raising step is preferably a temperature rising rate of 200 ° C./h or less. This is because if the rate of temperature increase is too high, the material to be heat treated may be distorted, or the temperature difference between the surface layer portion and the inside of the material to be heat treated will increase, resulting in variations in crystal grains depending on the part. This is because it becomes higher. A preferable temperature increase rate is in the range of 50 ° C./h to 150 ° C./h.

You may perform the temperature holding process which hold | maintains temperature more than once in the middle of the above-mentioned low temperature side hardening temperature rising process (FIG. 1 (2)).
By performing the temperature holding step, the temperature unevenness when the heat-treated material is heated is reduced, so that deformation is reduced. Further, the processing residual stress generated at the time of mold manufacture is removed by preheating, and there is also an effect of suppressing abnormal growth of crystal grains using residual strain as a driving force when passing through the transformation point by subsequent heating. In order to obtain this effect more reliably, the temperature holding step may be performed in a temperature range of A1 transformation point −200 ° C. to A1 transformation point −15 ° C. More preferably, it is the temperature range of A1 transformation point-70 degreeC-A1 transformation point-20 degreeC.
Note that the temperature holding time is intended to reduce the temperature unevenness when the heat-treated material is heated as described above, and therefore it is difficult to obtain the effect of reducing the temperature unevenness in a very short time. Therefore, it is preferable to set a time sufficient to reduce the temperature unevenness. Although the time cannot be generally determined depending on the weight and shape of the material to be heat-treated, it is preferable to hold it for about 0.5 to 5 hours from experience. If held for 0.75 hours or longer, the difference between the surface layer temperature and the internal temperature can be kept within 30 ° C., so it is preferable to hold it for 0.75 hours (45 minutes) or longer.

After the temperature holding step, the temperature range from the A1 transformation point to the A3 transformation point may be heated at a rate of 100 ° C./h or more as a high temperature side temperature raising step (FIG. 1 (3)). This is because when a new austenite crystal grain is produced from ferrite, if the heating rate is high, the nucleation density of the austenite is high due to the overheating effect from the equilibrium temperature, and the action of refining the crystal grain is obtained. is there.
As described above, the non-heat treated member can be adjusted to uniform crystal grains by adjusting the conditions of the temperature raising step up to the heating / holding step.

The following examples further illustrate the present invention.
From the mold material for hot dies having the composition shown in Table 1, a block of 300 mm (w) × 300 mm (l) × 300 mm (t) is cut out, and a first sample subjected to a first improved holding process, A second material for the second improved holding process was prepared. All the prepared alloys are JIS-SKD61 equivalent materials.
The alloy has an A1 transformation point of 850 ° C., an A3 transformation point of 895 ° C., an Ms point of 300 ° C., and a nose of the grain boundary carbide precipitation region of 700 ° C. to 20 minutes.

Quenching treatment was performed using the above sample. The temperature conditions up to the heating / holding step are as follows.
<First sample>
The first sample was inserted into a quenching furnace, and the temperature increase was started. The conditions of the low temperature side temperature raising step (1) were 150 ° C./h, and the temperature holding step (2) was performed at 800 ° C. for 4 hours. Then, it heated up to 1025 degreeC on 150 degreeC / h conditions as a high temperature side temperature rising process (3), and moved to the heating and holding process (4).
<Second sample>
The second sample was inserted into the quenching furnace and the temperature increase was started. The conditions of the low temperature side temperature rising step (1) were 200 ° C./h, and the temperature holding step (2) was performed at 800 ° C. for 2 hours. Then, it heated up to 1025 degreeC on 200 degreeC / h conditions as a high temperature side temperature rising process (3), and moved to the heating and holding process (4).

  The first improved holding step and measurement temperature applied to the first sample, and the second cooling step, and the second improved holding step and measured temperature applied to the second sample, and the second Table 3 shows the cooling process. The temperature is a result of measuring the vicinity of the surface and the inside (center portion) by forming a drilling hole in the sample and inserting a thermocouple thermometer.

After completion of the above steps, tempering was performed to 45HRC and the Charpy impact value was evaluated. The Charpy impact test was a 2 mm U notch test. Table 4 shows the Charpy impact values. The “comparative example” in Table 4 is the “No. This is a test result of 6 alloys (JIS SKD61 equivalent alloy), which is due to cooling by the conventional marquenching method. This comparative example (No. 6 of Patent Document 2) has almost the same thermal history as the example performed this time, and the quenching conditions are as follows.
The test piece was heated at 75 ° C./h (low temperature side heating step (1)) and held at 800 ° C. for 1 hour (temperature holding step (2)), and the temperature was increased to 1020 ° C. under the condition of 175 ° C./h. It was heated (high temperature side temperature raising step (3)), and moved to the heating / holding step (4). Thereafter, the temperature range of A3 to 600 ° C is cooled at a rate of 12 ° C / min, held at 400 ° C for 1 hour, and the temperature range of 400 ° C to 200 ° C is cooled at a rate of 10 ° C / min. went. Cooling is performed while controlling the amount of gas pressurization in an inert gas atmosphere.

  As shown in Table 4, it was confirmed that the one to which the quenching method of the present invention was applied had toughness superior to that to which the conventional marquenching method was applied. Moreover, although the sample size and weight are the same, the Charpy impact value inside the second sample is improved. This indicates that the grain boundary precipitation of carbides can be suppressed even inside the mold because the cooling rate inside the mold is increased.

FIG. 2 shows the measured values of the heat pattern after the first cooling step for the second sample, and the simulation result when the dipping and lifting are performed only once (adopting the first improved holding step). (Corresponding to the result) was plotted.
As shown in FIG. 2, it can be seen that the internal (center) cooling rate is faster when immersion and pulling are repeated. It can also be confirmed that the overall cooling rate is fast. From these results, it can be seen that the present invention in which the second improved holding step is particularly suitable for quenching a heavy mold.
In addition, as a result of cutting out the test piece for micro observation from the surface side of the 2nd sample and investigating the presence or absence of grain boundary precipitation, as shown in FIG. 3, the grain boundary precipitate could hardly be confirmed. Moreover, the dimensional change amount was about 0.5 mm at the maximum, and it was confirmed that the dimensional change could be suppressed.

  Since the mold quenching method of the present invention can improve the toughness of the mold, it can be expected to improve the life of the mold. Therefore, not only the mold but also the other application can be expected as a quenching method capable of improving toughness. In particular, the larger the heat-treated material, the better the improvement effect can be expected.

DESCRIPTION OF SYMBOLS 1 Low temperature side temperature rising process 2 Temperature holding process 3 High temperature side temperature rising process 4 Heating / holding process 5 First cooling process 6 Improved holding process 7 Second cooling process

Claims (5)

A heating / holding step of heating the mold and holding it in a temperature range from the A3 transformation point to 1150 ° C,
After the heating and holding step, the mold is immersed in an oil bath, and the mold surface is cooled at a rate of 80 ° C./min to 250 ° C./min by oil cooling, and the mold surface temperature is 700 ° C. or less. A first cooling step for cooling to a temperature exceeding the Ms point;
After the first cooling step, the mold is lifted from the oil tank to interrupt oil cooling, and the immersion and pulling are performed again in a temperature range where the surface temperature of the mold exceeds the Ms point, An improved holding step that repeats until the temperature difference between the surface and the interior is within 200 ° C .;
A second cooling step of cooling at a rate of 1 ° C./min to 50 ° C./min until the surface temperature of the mold reaches 200 ° C. after the improved holding step;
A mold quenching method characterized by comprising: The method for quenching a mold according to claim 1 , wherein the number of pulling-up times in the improved holding step is 3 or more. Wherein in the second cooling step, to any one of claims 1 to 2 temperature inside the mold, characterized in that the cooling at 1 ° C. / minute to 15 ° C. / min down to 250 ° C. from 400 ° C. The method of quenching the described mold. The second cooling step, the mold quenching method according to any one of claims 1 to 3, characterized in that an oil cooling. The mold quenching method according to any one of claims 1 to 4 , wherein the environment in the first cooling step, the improved holding step, and the second cooling step is a non-oxidizing atmosphere.