Today, with the increasing development of industrial technology, molds have become an important part of “high-precision” industrial production. The value of products produced by molds can be said to be ten times or even a hundred times the value of molds. Die technology is a precision process technology with very strong comprehensive technology. It involves the development and application of new materials, new technologies and new equipment. It is a systematic project composed of materials, metallurgy, electromechanical, computer, forging, heat treatment and other disciplines and processes . Products produced with molds have the characteristics of high efficiency, low consumption, and high precision, which are beyond the reach of other manufacturing processes. Nowadays, the mold manufacturing industry has gradually developed an interdependent relationship with the high-tech industry, and the level of mold manufacturing has become the main indicator to measure the national manufacturing process. For a country, manufacturing is a concentrated expression of the overall national strength and industrial technology level, and the development of mold manufacturing is its key. In response to this situation, my country has issued many policies to actively develop the mold manufacturing industry.
Generally speaking, the number of workpieces is the first factor to be considered when choosing a cold stamping die. If the number is small, it is not necessary to choose those materials with a longer service life, which can reduce the production cost. On the other hand, it is also necessary to consider the material of the material processed by the cold stamping die. Each material has a greatly different stamping property, and the applicable die material will also be different. From this aspect, the wear-resistant characteristics of metal materials have determined the use time of the stamping die itself. Under normal circumstances, the wear resistance of steel materials depends on the actual situation of the carbide hard spots inside the steel and the hardness of the steel material itself. When both of them can maintain high hardness, the more carbides contained in the material, the more More, the wear resistance will be more significant. On the whole, the basic order of wear resistance of metal stamping dies commonly used in industrial production is: hard alloy > steel-bonded hard alloy > high-speed steel > carbon steel. In addition, the influence of mold shape, mold thickness, mold accuracy, etc. on the mold material should be fully considered.
1 Overview of cold stamping
1.1 The basic concept of cold stamping
The so-called cold stamping specifically refers to the processing method of applying pressure to the used material with a stamping die on a press in a normal temperature environment to deform it to obtain the required standard parts. Since cold stamping is often used in sheet metal processing, it can also be called sheet stamping. Cold stamping has many characteristics, specifically: first, high dimensional accuracy.Since the current industries have basically adopted the assembly line production method, the required components need to be standardized, and the traditional manual operation can no longer meet the precision requirements. Cold stamping has improved in this regard.
The accuracy of the parts is improved to meet the production needs; secondly, the production efficiency is high. The use of unified standard components improves production efficiency, reduces the labor intensity of workers, improves product quality and enhances the competitiveness of enterprises; third, cost savings. Due to the high level of information technology used in cold stamping, it can be fully utilized in the process of material production and processing, which greatly reduces the generation of waste, improves the utilization rate of materials and saves costs to a certain extent, and can obtain greater benefits in market competition.
1.2 Development Status of Cold Stamping Dies
Nowadays, although my country’s stamping dies have achieved rapid development in terms of quantity and quality, there is still a certain gap compared with developed countries and actual national needs. Many precision, high-consumption and complex dies still need to pass Import to meet the needs, especially some high-end molds (automobile covering molds) are completely monopolized by developed countries. And those simple stamping dies have long been oversupplied in the market competition. According to relevant data, my country’s total import and export of molds in 2017 was about 7.5 billion US dollars, an increase of 11% year-on-year, of which the total import value was about 2 billion US dollars, a year-on-year increase of 3%. According to the type of mold, the highest import is injection molding rubber mold, accounting for 47% of total imports, followed by cold stamping molds, accounting for 39% of total imports. According to the import region, the main source countries of imported molds are Japan, Germany, my country’s Taiwan, Peng, Jinma tax zone, and the United States, accounting for about 75% of the total import. According to the import destination, the most imported ones are Jiangsu, Guangdong and Zhejiang, accounting for about 70% of the total import. Judging from the price of imported dies, the average price of stamping dies reached US$8,942, a year-on-year increase of 12%. Compared with the previous year, the gap has narrowed significantly. It can be seen that the proportion of my country’s imported molds is shrinking, which fully reflects the technological progress of my country’s mold industry.
2 Common metal materials for cold stamping dies
2.1 Carbon Tool Steel
Carbon tool steel not only has a relatively high output in my country, but also has a wide range of uses. This is because carbon tool steel has many incomparable advantages over other metal materials. First of all, the forgeability is good, and it is easy to forge into the desired shape; secondly, the softening speed after annealing is fast, so as to facilitate the subsequent processing; thirdly, the cutting effect is good, due to the low hardness of carbon tool steel, it is easy to cut; finally, Low price, which is also the main factor for the widespread use of carbon tool steel. However, it should be emphasized that carbon tool steel also has many problems, such as poor hardenability, easy deformation and even tempering. Therefore, in view of the above advantages and disadvantages of carbon tool steel, it is only suitable for making ordinary cold stamping dies. Due to the small size and simple shape of these molds, the deformation requirements are not high, so making such molds with carbon tool steel can save a lot of resources. But for large, complex, high-precision molds, carbon tool steel is out of date at this time.
2.1 High Chromium Steel
The main difference between high chromium steel and carbon tool steel is that it can show better wear resistance and hardenability. Because high-chromium steel itself is not prone to deformation and other characteristics, it is often used as high wear-resistant die steel, but compared with other high-speed steels, high-chromium steel is not very ideal in terms of bearing capacity. The main disadvantage of high-chromium steel is the deflection of carbides that often occurs. In actual stamping, it must be repeatedly forged to improve the average level of carbides in the material. Properties of steel molds.
2.2 High Speed Steel
Most of the high-speed steels that are often used now are forged from molybdenum elements, so high-speed steel also has excellent performance. Its most significant advantages are thermoplasticity and high toughness, so it has obtained a large development space. Cold stamping dies and large-scale industrial production have played an important role. In addition, high-speed hard steel developed based on high-speed steel has also been used to a certain extent. The purpose of the development of high-speed hard steel is to meet the needs of cutting materials that are difficult to cut. But there were some problems along the way. For example, poor toughness, difficult post-processing, and poor bending resistance. The main factor causing these problems is that in order to increase the hardness of high-speed hard steel, the carbon content must be increased, and when the carbon content reaches a certain level, over-burning will occur, resulting in a decrease in toughness.
2.3 Base steel
In view of the many problems of high-speed steel and high-speed hard steel, the solution in practical operation is to add a small amount of other components, and appropriately increase or decrease the carbon content in the high-speed steel, so as to reasonably improve its stamping properties. This is produced by a special process. The metal material is the base steel. The toughness of the base steel is strong, it has all the advantages of high-speed steel and high-speed hard steel, and its cost is far less than that of high-speed steel. For cold stamping dies, the advent of base steel is a revolution.
If only considering the two characteristics of wear resistance and hardness, hard alloys are much stronger than the above-mentioned cold stamping die metal materials, but in terms of bending resistance and toughness, the performance of hard alloys is much inferior. The best way to improve the hard alloy is to make a small amount of iron powder and alloy powder into a binder, tungsten carbide as the hard phase, and sintered by powder metallurgy to form a hard alloy. Because the material of hard alloy is still steel, it can be processed by cutting, welding and other measures.
3 Heat treatment process of cold stamping die
For a long time, the cost of molds in industrial production has been high, especially for cold stamping molds with higher precision requirements. The most commonly used process for deep processing of molds is heat treatment. Using this process can not only prolong the service life of the mold, but also effectively improve the performance of the mold. Therefore, it has great economic benefits and is widely used.
3.1 Vacuum method
The vacuum treatment in the heat treatment process can keep the external properties of the mold well, and the overall deformation is small. So why didn’t this happen? The main reason is that in the process of vacuum heating, the outside of the mold material is still not active, so there is no significant carbon separation phenomenon, and no oxide film is formed to affect cooling. Since the vacuum treatment can degas the outside of the mold, the mold material has good mechanical properties, and increasing the temperature will also enhance the bending resistance of the material. After the end of vacuum quenching, the toughness of the mold material is also enhanced.After scientific calculation, the vacuum-treated mold has a longer service life than the ordinary process-treated mold.
45%. Today, the vacuum treatment technology of cold stamping dies has tended to mature and has been widely used in actual industrial production.
3.2 Extreme cold method
As with vacuum treatment, extremely cold treatment of cold stamping dies is also possible
Its mechanical properties are optimized to a certain extent, and the service life of the mold can be extended. From an overall perspective, the extreme cold treatment of the mold material can be performed between quenching and tempering, or it can be set to operate after tempering. After the mold is extremely cold treated, its wear resistance and stability are significantly improved. Nowadays, extremely cold treatment is widely used in the process of cold stamping die making, and also plays an important role in hard alloy treatment.
3.3 Chemical method
Generally speaking, conventional chemical treatment can effectively treat the outside of the mold material. After chemical treatment, the wear and corrosion resistance of the mold can be effectively improved. Tempering the alloy mold at high temperature enables simultaneous nitrocarburizing. At present, the chemical treatment commonly used in industrial production mainly includes nitriding process. Among them, the ion nitriding process is the most advanced in the nitriding process, which can not only greatly reduce the nitriding time, but also obtain a better quality external nitriding layer, so as to make the cold stamping die wear-resistant, Corrosion resistance and other properties have been significantly improved.
3.4 Quenching method
In actual industrial production, the mold material can be treated with a method higher than the ordinary quenching temperature, and the mold material can also be treated with a method lower than the ordinary quenching temperature. After high/low quenching treatment, the carbon in the mold material can be significantly reduced. The content not only improves the performance of the mold material, but also prolongs the service life of the mold.
To sum up, in the information society, the industrial economy is gradually prospering. First, mass production greatly reduces production costs, and secondly, personalized customization also meets people’s diverse needs. Whether it is mass production or individual customization, molds play an extremely critical role. Although my country’s mold industry has made great progress in recent years from an overall point of view, there is still a big gap between the core technology level and developed countries. In order to meet the needs of national economic development and modernization, my country’s mold industry needs to speed up technological optimization. and localization process. Therefore, it should be based on the national conditions, focus on the development of core technologies in the mold industry, continuously increase the research on high-tech development and application, improve the ability of independent innovation, use informatization to promote mold manufacturing, and actively adopt new technologies to improve molds The overall level of the industry makes my country’s mold manufacturing gradually develop in the direction of high precision, complexity and high efficiency, which is a long-term task for the development of my country’s mold industry. In addition, with the rapid development of social economy, molds with various functions are more and more concerned by people. Mold manufacturing and improvement technology is not only an important problem facing current industrial production, but also a key point of technological development. The research on metal materials and heat treatment processes commonly used in cold stamping dies has crucial value and significance for the practical development of many fields in my country.
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