KR101386030B1 - Mold surface caburizing method - Google Patents

Abstract

A method for surface carburizing heat treatment of a mold is provided.
According to the present invention,
Primary preheating the SKD 61 mold material, followed by primary carburizing heat treatment at a temperature range of 970 ° C to 990 ° C, followed by primary cooling; Second preheating of the cooled material, followed by secondary carburization heat treatment at a temperature range of 970 ° C to 990 ° C, followed by secondary cooling; Heating the cooled material to homogenize the tissue in a temperature range of 1000 to 1050 ° C., and then quenching the material to a temperature range of 100 to 80 ° C .; Sub Zero the quenched material, and then tempering twice in a temperature range of 180 ~ 240 ℃; is configured to include.

Description

Surface carburizing method of mold {Mold surface caburizing method}

The present invention relates to a method for surface carburizing heat treatment of a mold, and more particularly, to surface carburizing heat treatment of a mold that can dramatically improve not only the surface hardness but also the wear resistance by appropriately controlling the carburizing process, the hardening process, and the post-heat treatment process. It is about a method.

In the production of molds, SKD61 (hot working) and SKD11 (cold working) are used as materials, and these have been processed by adding nitriding heat treatment after vacuum heat treatment. Conventionally, in the forging field, a method of heat treating a mold has been used to improve the life and workability of the mold, and the heat treatment method may be classified into hot forging and cold forging. In particular, in cold forging, various coating methods have been applied to the surface of a mold, and are widely applied in various fields. However, in terms of working temperature, in the case of warm and hot forging (approximately 850 ℃ ~ 1200 ℃), which is worse than cold forging, the working temperature is high. The lifetime of the product depends on the quality of the processed product. Many problems have been caused by the heat treatment methods developed so far, and various coating methods have not only been expensive but have not had a great effect.

In other words, as the heat treatment method of the mold material which has been applied to the hot and hot forging and die casting until now, the salt bath treatment method, the electric furnace treatment method, and the vacuum furnace treatment method are used. Austenitic by heating to a target temperature of about 1000 ℃ to 1050 ℃ to maintain a suitable time, and then quenched in a neutral salt bath (quenching) in the vicinity of about 500 ℃, and then cooled by air-cooled, and then about 500 ℃ Heat to ˜600 ° C. to perform tempering at least twice to complete heat treatment. In addition, the electric furnace heat treatment method sets a mold in a container in which a heater is embedded and maintains it by heating to a target temperature. In this case, nitrogen gas is introduced to prevent oxidation of a product in the furnace atmosphere. The heat-treated mold is quenched by air cooling or oil cooling, and then heated to about 500 ° C. to 600 ° C. to perform a tempering operation two or more times to complete the heat treatment. In addition, the vacuum furnace heat treatment method sets a mold to be heat-treated in a container in which a vacuum can be maintained, operates a vacuum pump to maintain an appropriate vacuum degree (10 to 10 torr), and when the power is supplied to a carbon heater, the mold is heated by resistance. It heats up to this target temperature. After maintaining a suitable time in this state, the quenching work is completed by adding and cooling nitrogen gas pressurized (2-4 bar) into the furnace, and again, the tempering work is performed twice or more by heating to about 500 to 600 ° C. To complete the heat treatment.

All of the above heat treatment methods are heated to the target temperature (quenching temperature), and the method of transferring heat in the furnace is convection or conduction when salt bath and electric furnace, and when using vacuum furnace, the difference between using radiation method and cooling method. Depending on the difference, there may be some differences in quality due to the degree of deformation and hardness after heat treatment.But basically, each heat treatment is different from the method of hardening by quenching after austenizing carbon and other alloy elements in the heat treated parts. There is no difference.

As a practical example, when the STD61 grade (JIS: SKD61) grade, which is widely used as a material of hot die, is heat treated by the above three kinds of heat treatment methods, the internal structure change of the steel is quenched with the dissolved carbon (about 0.35%). It is transformed into martensite (base tissue) structure and some residual austenite by work, and some carbon is combined with chromium and may exist in tissue as chromium-carbide carbide, but it is extremely small, and it is corroded by some corrosive liquid (nital etc.) Tissue photography is not well distinguished.

It is important here that the hardness of martensite in which carbon is transformed about 0.35% after heat treatment is only about HRc50 to 54. This is because the temperature of the workpiece to be contacted to the mold during hot work is high temperature (750 ℃ to 1200 ℃). Even though cooling water and lubricants are used, the martensite structure formed by the tempering effect is decomposed and the hardness of the mold decreases, causing wear and sintering of the mold. This, in turn, contributes to the cost increase due to the deterioration of workability of mold makers and the quality problems of products, and also a major cause of competitiveness deterioration. As described above, the three heat treatment methods cannot basically solve the wear resistance of the high temperature mold.

Meanwhile, another conventional technique is a technique of carburizing heat treatment in a PIT furnace as a product of SKD 11 material, but this technique has a problem that excessive carburization or decarburization occurs on the surface of the product, so that surface polishing of 0.2 mm or more after completion of heat treatment is performed. I'm doing it.

Moreover, only 11 wear resistance of SKD reduces the durability of the product, and the situation is complementing the wear resistance of the product by combining the carbide tips to the product.

Therefore, the situation that the technical development request for the surface heat treatment method of the mold that can solve the above limitations of the prior art has been continued.

Accordingly, an object of the present invention is to provide a method for surface carburizing heat treatment of a mold that can improve hardness and wear resistance of a mold by optimally controlling carburizing heat treatment conditions and post-heat treatment conditions. It is done.

According to an aspect of the present invention,

Primary preheating the SKD 61 mold material, followed by primary carburizing heat treatment at a temperature range of 970 ° C to 990 ° C, followed by primary cooling;

Second preheating of the cooled material, followed by secondary carburization heat treatment at a temperature range of 970 ° C to 990 ° C, followed by secondary cooling;

Heating the cooled material to homogenize the tissue in a temperature range of 1000 to 1050 ° C., and then quenching the material to a temperature range of 100 to 80 ° C .; And

Sub-zero the quenched material, and then, preheating at a temperature of 50 ~ 70 ℃ and then tempering twice in a temperature range of 180 ~ 240 ℃; relates to a surface carburizing heat treatment method of a mold.

In the present invention, the first preheat temperature is 710 ~ 730 ℃, and the second preheat temperature is preferably managed in the range of 790 ~ 810 ℃.

In the present invention, a spherical carbide can be formed in the surface carburized portion of the mold material through the two carburizing processes.

It is preferable to manage the total of the carburization time to 20 to 24 hours.

It is preferable to manage the cooling temperature at the time of the primary and secondary cooling in the range of 690 ~ 710 ℃.

The step of quenching,

Oil quenching for about 2-3 minutes; And gas quenching for about 50-70 minutes.

It is preferable to include a preheating process in the temperature range of 50 ~ 70 ℃ between the Sub Zero process and the Tempering process.

The present invention of the above-described configuration has the advantage that no excessive carburization or decarburization occurs and thus no separate polishing process is required in subsequent processes such as the prior art.

In addition, by optimizing the carburizing heat treatment process, spherical fine carbides may be formed inside the carburized material to have a high surface hardness of HRC 70.

In addition, by forming a fine carbide inside the carburized surface has the advantage that it can have a durability close to the cemented carbide, and it is a useful process that can simultaneously obtain productivity and cost reduction.

1 is a view showing a carburizing heat treatment process and a curing process according to an embodiment of the present invention.
2 is a view showing a post-heat treatment process according to an embodiment of the present invention.
Figure 3 is a photograph of the structure of the surface for the mold material produced by one embodiment of the present invention.
Figure 4 is a tissue photograph of the surface of the mold material prepared by another embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

First, in the present invention, the SKD 61 mold material is preheated first.

SKD 61 mold material refers to hot die steel and is presented in JIS standard. Specifically, in weight percent, C: 0.32 to 0.42%, Si: 0.82 to 1.20%, Mn: 0.5% or less, Cr: 1.50 to 5.50%, Mo: 1.00 to 1.50%, V: 0.80 to 1.20%, residual iron It has a steel material component including composition. In general, the use of SKD 61 steel is required to be less deformation after processing and heat treatment as it is used in the mold manufacturing.

In the present invention, the SKD 61 material is first preheated at a temperature range of 710 to 730 ° C, and then carburized first at a temperature range of 970 ° C to 990 ° C. This carburization process is performed in a conventional well known carburization heat treatment furnace, and carburization occurs on the surface of the SKD 61 material by diffusion of exposed carbon into the material.

At this time, in the present invention, it is preferable to manage the range of the carburization temperature to 970 ~ 990 ℃. This is because excessive carburization does not occur in this temperature range, and the carburization can be carburized to the maximum carburizing limit. And the depth of the carburized layer is preferably managed to about 2mm or so.

And the primary preheating temperature is preferably managed in the range of 710 ~ 730 ℃.

The carburized material as described above is subsequently subjected to primary air cooling. By this cooling, the carbon content of the surface layer of the material due to carburization becomes excessively unstable, and carbon precipitates in the grains.

At this time, in the present invention, it is preferable to manage the range of the cooling temperature in the range of 690 ~ 710 ℃.

Subsequently, in the present invention, the first cooled material is preheated to the second, and at this time, it is preferable to manage the range of the preheating temperature in the range of 790 to 810 ° C. Due to this cooling treatment, the carbon precipitated in the grains by primary cooling described above is grown by diffusion to form a sphere.

Subsequently, in the present invention, the preheated material is subjected to secondary carburization heat treatment at a temperature range of 970 ° C to 990 ° C. Through the carburizing heat treatment process, the surface of the material is carburized again and at the same time, due to the spherical precipitate formed during the second preheating, grain growth due to diffusion is suppressed, and the grains of the material surface as a whole become finer. That is, the grain refinement of the surface of the material is made finer than the first carburizing treatment due to the finer precipitate action. Thereafter, the second carburized material is subjected to a second air-cooled cooling process, and it is preferable to manage the range of the cooling temperature at a range of 690 to 710 ° C.

As described above, in the present invention, based on the primary carburizing heat treatment and the secondary carburizing heat treatment, the carburizing heat treatment may be repeated a plurality of times as necessary. This can be freely controlled at an appropriate number of times in consideration of the thickness of the carburized layer.

In addition, the total residence time of the carburizing heat treatment is preferably managed to 20 to 24 hours.

Subsequently, in the present invention, the second cooled material is heated to homogenize the tissue in a temperature range of 1000 to 1050 ° C. In other words, the entire carburized surface structure is homogenized by Austenizing treatment at such a temperature range, and this homogenization is for achieving homogeneity of the material surface structure obtained in a subsequent process.

Since in the present invention, the surface is homogenized material is quenched in a temperature range of 100 ~ 80 ℃. In this quenching process, the microstructure of the carburized layer on the surface of the material changes to martensite structure and some residual austenite is present.

At this time, the step of quenching, step of quenching (oil) for about 2-3 minutes; And gas quenching for about 50-70 minutes.

Subsequently, in the present invention, the quenched material is subjected to sub zero. This sub zero treatment transforms the residual austenite structure remaining in the surface microstructure of the material into a martensite structure. This sub zero treatment prevents dimensional errors caused by residual austenite tissue, thereby providing dimensional accuracy.

In the present invention, the sub zero treatment temperature can be managed in the range of approximately -180 ° C.

Subsequently, in the present invention, the sub zero treated material is tempered twice in a temperature range of 180 to 240 ° C. Tempering refers to a heat treatment that reheats steel with high hardness by quenching to an appropriate temperature and cools, softens and stabilizes the structure in the air, thereby giving toughness to the material and at the same time eliminating internal stresses. The surface texture of can be changed to a tempered martensite structure to give the material better toughness.

Meanwhile, in the present invention, a process of preheating the material may be provided between the sub zero treatment and the tempering heat treatment as necessary. This effectively prevents distortion caused by rapid heating of the Sub Zero treated material.

In addition, in the present invention, it is preferable to manage the total time of the Sub zero treatment and Tempering treatment to 13 to 17 hours.

As described above, the present invention forms a spherical fine carbide inside the carburized material surface by appropriately controlling the carburizing heat treatment process and the post-heat treatment process, and can not only have a high surface hardness of HRC 70 but also a mold material having excellent surface toughness. Can be prepared.

Hereinafter, the present invention will be described in detail by way of examples.

(Example)

After preparing the SKD61 steel grade, the heat treatment as shown in Figs. That is, after preheating at 720 ℃, the first carburizing treatment in the carburizing heat treatment furnace at 980 ℃ and primary cooling to 700 ℃. The cooled material was secondly preheated at 800 ° C., and then subjected to secondary carburization heat treatment at 980 ° C., and then secondarily cooled to 700 ° C. After heating to 1030 ℃ and Austenizing treatment, and then quenched at a temperature of 90 ℃ through oil cooling for 2 minutes and gas cooling for 60 minutes.

The quenched material was sub zero treated at -180 ° C. Sub zero treated material was preheated to 60 ° C and then tempered twice at 200 ° C and then cooled.

The hardness of each depth was measured for the surface of the carburized material manufactured as described above, carburizing heat treated to a depth of 1 mm and finishing the post-heating treatment, and is shown in Table 1 below. On the other hand, Figure 3 is a photograph of the surface texture of the material used for the measurement of hardness (Hv (300g)).

Depth (mm) Hardness 0.05 1013 0.1 1021 0.3 971 0.5 919 0.7 866 0.9 858 1.1 812 Deep 812

In addition, the hardness of each of the carburized material prepared as described above, carburizing heat treated to a depth of 2mm on the surface and the post-heat treatment was finished, and shown in Table 2 below. On the other hand, Figure 4 is a surface texture picture of the material used for the measurement of hardness (Hv0.3).

Carburizing Depth 0.1 765 0.3 784 0.5 777 0.7 772 0.9 770 1.1 755 1.3 724 1.5 697 1.7 675 1.9 667 2.1 652 Deep 650

As shown in Table 1 and Table 2, the hardness of the surface of the mold material can be seen that the higher the hardness toward the surface.

As described above, the present invention is not limited to the above-described specific preferred embodiments, and various modifications can be made by those skilled in the art without departing from the gist of the present invention claimed in the claims. Of course, such changes will fall within the scope of the claims.

Claims (8)

Primary preheating the SKD 61 mold material, followed by primary carburizing heat treatment at a temperature range of 970 ° C to 990 ° C, followed by primary cooling;
Second preheating of the cooled material, followed by secondary carburization heat treatment at a temperature range of 970 ° C to 990 ° C, followed by secondary cooling;
Heating the cooled material to homogenize the tissue in a temperature range of 1000 to 1050 ° C., and then quenching the material to a temperature range of 100 to 80 ° C .; And
Sub-treating the quenched material, and then tempering twice in a temperature range of 180 to 240 ° C.,
The first preheating temperature is 710 ~ 730 ℃, and the second preheating temperature is a surface carburizing heat treatment method characterized in that the range of 790 ~ 810 ℃. delete The method of claim 1, wherein a spherical carbide is formed on the surface carburized portion of the mold material through the two carburizing steps. The method of claim 1, wherein the total of the carburization heat treatment time is 20 to 24 hours. The method of claim 1, wherein the cooling temperature during the first and second cooling ranges from 690 to 710 ° C. According to claim 1, wherein the step of quenching, the step of oil quenching 2-3 minutes; And a step of gas quenching for 50 to 70 minutes. The method of claim 1, further comprising a preheating step in a temperature range of 50 to 70 ° C between the subzero treatment and the tempering step. The method of claim 1, wherein the total time of the subzero treatment and the tempering treatment is 13 to 17 hours.

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