JPH11310821A - Method for quenching metallic mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the toughness, and the heat crack resistance with the setting of high hardness and the service life of a metallic mold by executing cooling of the metallic mold to a specific temp. range in a first process for cooling with the mixed coolant of liquid and gas and in a second process for cooling with the liquid coolant. SOLUTION: The metallic mold is heated to a prescribed temp. for obtaining uniform austenitic structure and successively, quenching is applied. Thereat, in at least a part of the temp. range of 950 deg.C to 750 deg.C, the cooling is applied by using the mixed coolant of the liquid and the gas. As the mixed coolant, desirably, water and air are used and the cooling is executed by spraying. Since the cooling is dependant on the diamension and the shape of the metallic mold, the coolant quantities of the liquid and the gas are controlled to cool the metallic mold with the optimum cooling capacity. Successively, in the temp. range of 400 deg.C to 250 deg.C, the cooling is applied by using the liquid coolant. The cooling of the liquid used in this second cooling process, is executed by using an oil and the cooling is desirable to be rapid cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for quenching a mold having excellent toughness.

[0002]

2. Description of the Related Art A quenching step is indispensable in the production of die casting dies and hot forging dies. Quenching is required to be cooled as rapidly as possible so as not to form a brittle bainite structure. Therefore, quenching by oil cooling is usually performed. However, large molds are liable to crack due to strain during cooling, and in many cases, blast cooling that allows formation of bainite to some extent is employed.

[0003] Recently, Japanese Patent Publication No. 6-104851 discloses a process of heating a hot die steel to an austenitizing temperature, and air-cooling or blasting the heated hot die steel at a cooling rate toward a nose in a bainite region. A step of cooling to a combined cooling change temperature by cooling, and a step of performing rapid cooling by oil cooling or the like at a cooling rate of avoiding a nose in the bainite region, the hot die steel being air-cooled or blast-cooled. A mold quenching method has been proposed. This mold cooling method does not increase the cooling rate in the high temperature region where heat treatment distortion is a problem, and adapts quenching treatment on the low temperature side where the formation of bainite structure is a problem, and a mold with high toughness can be obtained. Therefore, the life of the mold is also improved.

[0004]

In recent years, molds have tended to become larger and more complex in shape, and some molds have required further improvement in toughness. According to the study of the present inventor, the above-mentioned Japanese Patent Publication No. Hei 6-104851 was extremely effective in improving toughness. Further investigations revealed that carbide precipitation caused a decrease in toughness in a high temperature range during quenching. An object of the present invention is to provide a method of quenching a mold having better toughness.

[0005]

DISCLOSURE OF THE INVENTION The present invention has been accomplished by studying a quenching method capable of suppressing precipitation of carbides affecting toughness, suppressing formation of bainite structure, and suppressing cooling cracks. did. That is, 950
A first cooling step of cooling at least a part of the temperature range from 700C to 700C with a mixed refrigerant of liquid and gas;
This is a method of quenching a mold in which quenching is performed in a second cooling step in which cooling is performed in a range from to 250 ° C. with a liquid refrigerant.

Preferably, the cooling in the first cooling step is a method of quenching and cooling the mold, wherein water and air are blown.

[0007] More preferably, there is provided a method for quenching and cooling a mold, wherein the liquid refrigerant used for cooling in the second cooling step is oil.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor first conducted intensive studies on the effect of the structure of a mold material on toughness, and found that the austenite structure obtained at the time of heating to a quenching temperature had grain boundaries (hereinafter referred to as former austenite). (Referred to as crystal grain boundaries), and the effect of the coarsened bainite structure was found to be significant. Furthermore, the following conclusions were obtained as a result of studying the precipitation of carbides and the formation of bainite structure at the prior austenite grain boundaries. Since the precipitation of carbides on the prior austenite grain boundaries occurs during the cooling process from 950 ° C. to 700 ° C., it occurs when the cooling rate is low.

In the conventional cooling such as air cooling or cooling using only gas as a cooling medium, the cooling rate is slow to suppress the precipitation of carbides, particularly in a large mold or a complicated cavity. In some cases, toughness may be reduced due to the carbide precipitation. And by rapidly cooling this temperature range at a speed no longer than blast cooling,
It has been found that it is necessary to cool with a mixed refrigerant of liquid and gas in order to suppress the precipitation of carbides and suppress cooling cracks.

The bainite structure is rapidly cooled in a temperature range of 400 ° C. to 250 ° C. during the quenching and cooling process.
It was particularly important to avoid bainite nose, and it was also found that the cooling speed was insufficient with blast cooling or the like, resulting in a large decrease in toughness. Then, it has been found that by rapidly cooling the specific temperature range of 400 ° C. to 250 ° C., formation and coarsening of a bainite structure can be suppressed. In other words, to improve toughness,
It has been found that rapid cooling of the above-mentioned specific temperature range is effective in the quenching and cooling process.

Hereinafter, the present invention will be described in detail. First, the mold is heated to a predetermined heating temperature in order to obtain a uniform austenite structure, and subsequently subjected to quenching. Here, in the present invention, at least a part of the temperature range from 950 ° C. to 700 ° C. is cooled with a mixed refrigerant of liquid and gas. This is because the precipitation of carbide at the former austenite grain boundary is 950 ° C. to 70 ° C.
This is because in a temperature range of 0 ° C., the precipitation of carbides, which is a factor causing a decrease in toughness, is suppressed. For this reason, in air cooling with a slow cooling rate or cooling using only gas as a refrigerant, carbides are precipitated and the toughness is reduced.
Further, simple water cooling, oil cooling, or the like results in too high a cooling rate, resulting in a large heat treatment distortion, and in severe cases, cracks.

Therefore, in the cooling in the first cooling step of the present invention, at least a part of the cooling is performed by a mixed refrigerant of a combination of a liquid and a gas. Here, the mold to which the present invention is applied is not uniform in shape, and the cooling rate varies greatly depending on the size and shape. In particular, the cavity surface of the mold has a complicated shape having a corner portion having a small radius of curvature. Thus, the setting of the upper limit of the cooling rate of the mold depends particularly on the shape.

For this reason, in the present invention, the mixed refrigerant used in the first cooling step of the present invention is preferably cooled by spraying water and air, based on the size of the mold and the shape of the cavity surface. Here, since the cooling of the mold depends on the size and especially the shape, it is necessary to cool with the optimal cooling capacity for the mold. It is important to choose cooling methods to avoid. For example, by controlling the amount of liquid or gaseous refrigerant in accordance with the size and shape of the mold, an optimal cooling capacity can be obtained.

Next, in the present invention, the second cooling step 400
Cooling is performed in a temperature range from ℃ to 250 ℃ with a liquid refrigerant. The reason is as follows. In a temperature range of 400 ° C. to 250 ° C., if a cooling with insufficient cooling speed such as blast cooling is used, a coarse bainite structure cannot avoid bainite nose due to a low cooling speed and greatly reduces toughness. . That is, the toughness can be improved by rapidly cooling the cooling rate in this temperature range using a liquid refrigerant.

As the liquid refrigerant used in the second cooling step of the present invention, it is desirable to perform rapid cooling by oil cooling.
The reason for this is that the above cooling rate can be sufficiently obtained by oil cooling, and the cooling rate becomes excessively high as in water cooling or the like, and there is little occurrence of cracking during quenching cooling. By using these cooling methods, the mold can be cooled uniformly and more excellent toughness can be imparted.

[0016]

EXAMPLE In this example, to explain the effect of the present invention,
The die casting mold of the present invention and a comparative die casting mold were manufactured and their characteristics were compared. Each of these molds of the present invention and comparative molds uses SKD61 as a material, and the approximate dimensions are 350 mm × 310 mm × 170 m
m. The quenching of the mold of the present invention is performed at a quenching temperature of 10
After cooling from 30 ° C to 500 ° C by spraying water and air, oil cooling was performed at 500 ° C or less, and air cooling was performed at 150 ° C. The quenching cooling of the comparative mold was performed only by oil cooling. FIG. 1 shows the respective quenching cooling curves.

The comparative mold No. 1 which is a conventional mold is used. In the case of mold No. 2 of the present invention, cracking occurred at the corner portion having a small radius of curvature during quenching and cooling, making it unusable. In No. 1, no crack occurred during quenching and cooling. After the above-mentioned quenching treatment, the mold of the present invention is subjected to a
Conditioned to HRC. In addition, the comparative mold is also used for comparing the characteristics.
By the same tempering treatment, it was tempered to 45 HRC. Then, test pieces were collected from the mold of the present invention and the comparative mold, and 2
Table 2 shows the results of the mmU Charpy impact test. From this result, it is understood that the mold of the present invention has the same toughness as the oil-cooled mold. Further, when the microstructure of the test piece was observed, no precipitation of carbides was observed at the former austenite grain boundary in any of the mold of the present invention and the comparative mold. From the above results, it can be seen that according to the quenching and cooling method of the present invention, while preventing cracks during quenching and cooling, grain boundary precipitation of carbides can be suppressed, and a mold having excellent toughness can be obtained.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

According to the mold and the quenching and cooling method of the present invention, the toughness can be greatly improved, and the hardness of the mold can be set higher than before, so that the heat crack resistance is also reduced. Excellent, it is possible to improve the life of the mold.

[Brief description of the drawings]

FIG. 1 is a diagram showing a quenching cooling curve in an example of the present invention.

Claims (3)

[Claims] 1. A first cooling step of cooling at least a part of a temperature range of 950 ° C. to 700 ° C. with a mixed refrigerant of a liquid and a gas, and a cooling step of cooling a temperature range of 400 ° C. to 250 ° C. with a liquid refrigerant. A quenching method for a mold, wherein quenching is performed in a second cooling step. 2. The method according to claim 1, wherein the cooling in the first cooling step is performed by blowing water and air. 3. The method according to claim 1, wherein the liquid refrigerant used for cooling in the second cooling step is oil.