JP2006342377A - Method for quenching large-sized die - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method where the large-sized die for hot forging, hot pressing, die casting or hot extrusion, e.g., made of SKD61 steel is quenched while avoiding quench cracks so as to obtain a die combining low heat treatment strain and high toughness. <P>SOLUTION: Cooling continued to heating to a quenching temperature is performed in such a manner that the cooling rate at a position in which the cooling is latest, i.e., the center of the thickest part in the die is controlled to 20 to 5°C/min in a high temperature region from a quenching temperature to 600°C, and is controlled to 1 to 15°C/min in a low temperature region from 400 to 200°C. It is preferable that, in the meanwhile where the surface temperature of the die lies in a middle temperature region from 400 to 600°C, the die is temporarily held to the temperature in the region, or is annealed, thus the cooling is progressed in such a manner that the temperature difference does not exceed 300°C between the surface and the inside of the die. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an improvement in a quenching method for a large mold. Here, “large molds” are typically large and heavy, such as hot forging dies, hot press dies, die casting dies, hot extrusion dies made of JIS-SKD61 steel. This refers to molds ranging from several hundred kilograms to several tons. The “slowest cooling part” is considered to be the center of the thickest part of the mold.

The molds used in the above hot condition are used after being subjected to heat treatment such as quenching and tempering, but the molds are brought into contact with and separated from the heated forging material or molten metal. Since the surface is repeatedly heated and cooled, a heat check often occurs. As a measure to prevent such heat check, it is effective to increase the hardness of the mold, but the mold that is too hard has low toughness and poor impact resistance, so it accompanies the heat check part and the mold shape. Large cracks may occur starting from the notch. Therefore, a heat treatment method is desired that can provide high impact resistance even when the hardness is increased. From such a viewpoint, as a result of investigating the relationship between the cooling rate at the time of quenching the mold and the impact resistance, it was found that the faster the cooling rate, the better the impact resistance.

On the other hand, in recent years, there has been a tendency to improve production efficiency, reduce weight, and reduce costs by integrally manufacturing parts that have been conventionally manufactured by combining a plurality of members from the beginning. Along with that, molds for manufacturing parts are also becoming larger. When such a large metal mold is reinforced by quenching to improve impact resistance, there arises a problem that a fire crack is generated or a heat treatment distortion is large and the mold cannot be used as a mold. For this reason, in large dies, cooling during quenching must be performed under relatively mild conditions. In this case, the quenching effect is lowered, and it is difficult to achieve both high toughness while achieving prevention of cracking and low distortion.

As a means to overcome this situation, the distortion is reduced by gradual cooling in the high temperature region during quenching, but the quenching is completed in a short time without contact with the bainite nose given by the CCT (low temperature region continuous cooling) curve. By doing so, it has been proposed to improve the impact resistance by preventing the formation of bainite and to achieve both low distortion and impact resistance (Patent Documents 1 and 2). However, since the bainite nose of hot die steel typified by SKD61 exists around 30 to 60 minutes, it is necessary to increase the cooling rate in the high temperature range to a certain extent in order to cool it by avoiding the formation of bainite. Therefore, it is not desirable to sufficiently reduce the heat treatment strain.

Heating up to the quenching temperature is performed in a reduced pressure heating chamber, cooling in a high temperature region is performed in an inert gas atmosphere, particularly a pressurized atmosphere, and when the mold surface reaches 900 to 500 ° C., it is removed from the furnace and oil cooled. There is also an attempt to achieve both a low strain and a high impact value while preventing decarburization (Patent Document 3). However, this method requires changing the furnace atmosphere at a high temperature, and there is a risk of shortening the life of the furnace body. Heating and cooling are preferably performed consistently by heating in an atmospheric furnace and cooling in the air, or by heating in a vacuum furnace and cooling in an inert gas or its pressurized atmosphere.

There has also been considered a method in which the high temperature region is cooled by a blast and the low temperature region is cooled by introducing it into a polymer solution to increase the cooling rate to ensure impact resistance (Patent Document 4). This measure is effective in improving the impact resistance, but on the other hand, the cooling capacity of the polymer liquid is larger than that of the quenching oil, so that the risk of quenching cracks is increased in large molds.
JP 6-104851 JP-A-8-225830 JP 2001-181735 A JP-A 98-182948

In view of the current state of such technology, the inventors have investigated the relationship between the cooling rate during quenching and the impact value, quenching strain, and quench cracking, and obtained the following knowledge.

1) The effect of the cooling rate on the impact value is small in the high temperature region, and in the region of 600 ° C. or higher, the cooling rate of the slowest part is 5 ° C./min or higher, that is, at a rate at which pearlite precipitation does not occur. The impact value is not significantly reduced. However, the influence is large in the low temperature region, the impact value is low when the cooling rate is low, and the impact value is high when the cooling rate is high. At this time, if the cooling rate of the slowest part of cooling is 1 ° C./min or less, the impact value is greatly reduced, so it is necessary to ensure this limit. That is, even if the high temperature region is cooled so as to be slower than the bainite nose of the CCT curve in order to reduce heat treatment distortion, a high impact value can be obtained by sufficiently cooling the low temperature region.

2) Burn cracking is more likely to occur as the cooling speed in the low temperature region increases with a large mold. This is due to the time difference between the start and end of the martensite and bainite transformation between the mold surface and the inside. The greater the time difference, that is, the greater the temperature difference between the surface immediately before quenching and the center, the easier it is to crack. It is. Moreover, the larger the mold, the larger the temperature difference between the inside and outside in the stage immediately before the rapid cooling in the low temperature region, even if the high temperature region is gradually cooled. As a result of experiments on various cooling patterns, phenomena such as pearlite precipitation and bainite transformation are remarkably slow in the temperature range of 400 to 600 ° C, and even if kept in this temperature range for an industrially finite time, the impact value is affected. I found that there was no.

The object of the present invention is based on the above-mentioned knowledge obtained by the inventor and is a method for obtaining a mold having both low heat distortion and high toughness by performing quenching while avoiding quenching cracks in the quenching method for large molds. Is to provide.

The large mold quenching method of the present invention that achieves the above-mentioned object basically includes the cooling to the quenching temperature followed by the cooling at the slowest position, that is, at the center of the thickest part of the mold. By carrying out the cooling rate of 20 to 5 ° C./min or higher in the high temperature region from the quenching temperature to 600 ° C. and 1 to 15 ° C./min in the low temperature region from 400 ° C. to 200 ° C. Is a quenching method characterized in that a mold having a low strain and a high toughness is obtained.

When a large mold is quenched by the quenching method of the present invention, no cracking occurs during quenching, and a large mold having high toughness can be obtained with minimal heat treatment distortion.

Hereinafter, the above-described quenching conditions will be described. First, in the high temperature region, that is, in the range from the quenching temperature to 600 ° C., 20 to 5 ° C./min or more is the reason that the cooling rate does not significantly affect the impact value in this temperature region as described above. Since the decrease in impact value becomes significant when the deposition rate is less than 5 ° C./min, this cooling rate is the lower limit.On the other hand, regarding the heat treatment strain, it is preferable to cool as late as possible because the strain increases when cooled too quickly. This is a limitation based on the reason that 20 ° C./min is the upper limit from this viewpoint.

Next, the cooling rate between 400 ° C. and 200 ° C. is set to 1 to 15 ° C./min. As described above, the magnitude of the impact value is proportional to the cooling rate in this region. Therefore, we want to cool as quickly as possible, but on the other hand, if the cooling rate is too high, cracks will occur based on the temperature difference between the surface of the mold and the inside, so in order to avoid it, the upper limit is 15 ° C./min. From the viewpoint of the limit that the impact value is not too low, 1 ° C./min was set as the lower limit. It is preferably 5 ° C./min or more.

In the course of cooling, while the mold surface temperature is in the intermediate temperature region lower than 600 ° C. but higher than 400 ° C., the mold is temporarily held at the temperature of this region or slowly cooled, It is preferable to proceed with cooling so that the temperature difference between the mold surface and the slowest cooling portion does not exceed 300 ° C. At 400 to 600 ° C., the start time of the pearlite transformation and the bainite transformation is very slow, and holding in this temperature region can reduce the temperature difference between the surface and the inside of the mold without causing a change in structure. Is possible.

Temporary holding or gradual cooling in the temperature range of 400 to 600 ° C., as described above, prevents the temperature difference between the mold surface and the slowest part of cooling from exceeding 300 ° C. If the difference is large, the time difference until the phenomenon that the center portion is transformed into martensite or bainite by the subsequent cooling process and becomes extremely brittle and then the center portion is transformed into martensite or bainite and expands is increased. This is because burning cracks are difficult to avoid.

One aspect of carrying out the quenching method of the present invention is that heating to a quenching temperature is performed using an atmospheric furnace at atmospheric pressure, and cooling is performed by air cooling or blast cooling in the atmosphere in a high temperature region, and in a low temperature region. A cooling rate of 1 to 15 ° C./min is ensured by introducing the oil into 40 to 200 ° C. oil. In order to set the cooling rate in the low temperature range to 1 to 15 ° C./min, it is necessary to appropriately adjust the temperature of the quenching oil within the range of 40 to 200 ° C. If the temperature of the quenching oil is less than 40 ° C., the cooling becomes abrupt and the cracking is likely to occur. If the temperature is higher than 200 ° C., the required cooling rate may not be obtained.

Heating to the quenching temperature may be performed using the above-described furnace in an inert gas atmosphere at atmospheric pressure, or may be performed in a vacuum. Cooling can be selected in the air or the above-described oil quenching, but may be performed in an inert gas atmosphere under atmospheric pressure or under pressure.

As shown in Fig. 1, the SKD61 steel spheroidized annealing material is cut into a cube of 300mm x 300mm x 300mm, and a test piece is prepared by machining a groove with a width of 50mm and a depth of 50mm on one side. After heating to 1030 ° C., quenching was performed under various conditions. At this time, the temperatures of the surface and the center of the test piece were measured with thermocouples. The cooling conditions are listed in Table 1.

Then, it tempered to 48HRC, and the presence or absence of a tempering crack (observation of the corner of a groove | channel), the Charpy impact value of the center part, and the amount of quenching distortion were measured. The Charpy test piece is provided with a 2 mm U-notch in a direction perpendicular to the forging direction. The method for measuring the quenching strain is as shown in FIG. The measurement results are shown in Table 1 together with the heat treatment method. There are the following two heat treatment methods.
A: Heating to a quenching temperature in a vacuum or an inert gas atmosphere, followed by cooling in an inert gas atmosphere or its pressurized atmosphere.
B: Heated to quenching temperature in an inert gas atmosphere at atmospheric pressure, removed from heating furnace, air cooled or blast cooled in air. The low temperature range is quenched in oil at 200-40 ° C.

The data in Table 1 shows the following facts. That is, in the example in which quenching according to the conditions of the present invention was performed, the result that the impact value was high and the distortion was small was obtained without causing any cracking. In Comparative Example A, the heat treatment strain is large because the cooling rate in the high temperature region is too fast, and the temperature difference between the surface and the interior immediately before the low temperature region quenching (measured at 400 ° C.) is large. It was. In Comparative Example B, since the cooling rate in the high temperature region was too slow, pearlite was precipitated during cooling, and the impact value was lowered. In Comparative Example C, since the cooling rate in the low temperature region is too slow, a sufficient impact value cannot be secured.

It is a figure which shows the shape of the test piece used in the Example of this invention, Comprising: A is a top view, B is the side view seen from the downward direction. Explanatory drawing which shows the measuring method of the heat processing distortion amount using the test piece of FIG.

Claims (4)

It is a quenching method for large molds, and the cooling following the heating to the quenching temperature is performed, the cooling rate of the slowest part of the cooling is 20 to 5 ° C / min or more, 400 ° C in the high temperature region from the quenching temperature to 600 ° C A quenching method characterized in that a low distortion and high toughness die is obtained by avoiding quenching cracks by carrying out so that the low temperature range from 1 to 200 ° C. is 1 to 15 ° C./min. In the course of cooling, while the mold surface temperature is in the intermediate temperature region lower than 600 ° C. but higher than 400 ° C., the mold is temporarily held at the temperature of this region or slowly cooled, The quenching method according to claim 1, wherein the cooling is advanced such that the temperature difference between the mold surface and the slowest cooling portion does not exceed 300 ° C. Heating to the quenching temperature is performed using an atmospheric furnace at atmospheric pressure, and cooling is performed by air cooling or air blast cooling in the air in the high temperature region, and by adding to oil at 40 to 200 ° C. in the low temperature region. The hardening method of Claim 1 or 2 implemented so that the cooling rate of 15 degree-C / min may be ensured. The quenching method according to claim 1 or 2, wherein heating to a quenching temperature is performed using a furnace in a vacuum or an inert gas atmosphere, and cooling is performed in an inert gas atmosphere at atmospheric pressure or under pressure.

Images