The chrome-plated steel sheet, which is called chrome plate for short, is a new type of can-making material that is developed due to the fact that the metal tin resource is small and the cost is high. The United States, Europe and Japan have more and more applications, replacing some of the tinplate materials, China's relatively few applications, which requires the correct understanding of this material, from the technical and ideological analysis.
First, the development and characteristics of chrome plate
Chromium is a kind of hard, silvery white shiny metal, hardness, wear resistance, matrix binding is very good, heat resistance is also higher, the potential is negative than iron, but the potential is more positive than iron after passivation. As early as the 20th century, the chrome plating process was used for industrial production. In the United States, in 1940, chromium plating plates were researched, followed by chromic acid chemical immersion treatment and electrolytic chromic acid treatment, and transferred to mass production. After living in Japan from 1955 onwards, the chromate passivation method was studied and many results were obtained.
For example, the Hitop chrome plate of the Dongyang Steel Company entered industrial production in 1961; Fuji Steel also produced Con Cupper chrome plates in succession; in 1972, Japanese alloy steel pipes produced Bright Coat. Chrome plate. European countries also successively produced chrome plates in the 1970s. The original plate and the tin plate of the chrome plate are the same, they are low-carbon steel plates, but the chrome plating layer is very thin (<1.3μm=. The process is the same as that of the tin plate. Therefore, the original tin plating line is generally only required. Plus retrofit, with the addition of a chrome-plated bath for tin or chrome plating.
The chrome plated structure has four layers: a steel substrate, a metallic chromium layer, a hydrated chromium oxide layer, and an oil film. Since the substrates are the same, the mechanical properties are the same as tinplates, but there are some unique features in their application:
1. The chrome plate has strong adhesion and adhesion to organic paint is 3 to 6 times stronger than tin plate. Sulfidation corrosion resistance is also stronger than tinplate.
2, chrome plate can not be soldered, can only use lap resistance welding or bonding.
3, the cost is about 10% lower than the tin plate, the appearance of gloss is not as good as the tin plate.
4. Corrosion resistance is lower than that of tinplate. The coating is thin and the pinhole rate is high. Therefore, the inner and outer surfaces must be coated when using.
5, thin coating and poor toughness, cans easily broken, so it is not appropriate to redial cans, can be used for deep-impregnated tanks.
6, chrome plate high temperature resistance, generally 5000C color and hardness are no significant changes, to 7000C only began to soften.
Second, the main issues of application of chrome plate in food can making
From the above understanding, there are mainly two technical problems that restrict the application of chrome plating in the food can making industry: one is the corrosion resistance problem. This problem can be studied from the improvement of the chrome plating technology and the improvement of the post-plating paint. It is important to note that any technical improvements must take into account the cost to maintain the advantages of the chrome plate. For the moment, the corrosion resistance of chrome plate is not as good as that of tin plate, but it also has certain corrosion resistance. It is suitable for weak acidic foods and packaging.
This kind of food market is very large, such as meat, fish, poultry, dairy products, vegetables, carrots, wrist beans, beets, potatoes and various beverages, soups and so on. If the coating is strictly technically guaranteed, some acidic foods with a pH of 3.7 to 4.5 can also be packaged in chrome plates, such as fruits, kimchi, and the like. It is worth mentioning that the corrosion resistance of chrome plate cannot be studied. The general concept of chromium does not have good corrosion protection for iron. The chrome plate is a cathodic coating and cannot play an electrochemical protective role. The porosity is high. Under the circumstances, the alloy effect between the steel substrate and the chromium layer is worth studying. Some people tested the corrosion resistance of the chromium steel in seawater and found that in the short-term (2 years) soaking, the chromium has an effect on the corrosion resistance of the carbon steel; but the long-term ( After more than 2 years) soaking is not only useless but also accelerates corrosion.
The second is the can manufacturing process. The chrome plating has a thin coating and poor toughness. It lacks the flushing and lubricating agent like the tin, so the flushing process needs careful study. At present, it is feasible to use deep-drawing process for two-piece cans. Since it is impossible to solder, three-piece cans must be welded or bonded. If a tinplate chrome plate is used in combination, tinplates for can bodies and plated plates for can bottom covers are also an application. In addition, the sealing of the can body and the lid is also not suitable for the double curling process.
Third, the chrome plate application thinking and understanding
The above analysis shows that the application of chrome plated in food packaging has its limitations, but it also has its own characteristics and advantages, and it has a relatively broad market, so it can be worthy of promotion and development. However, as far as China's current reality is concerned, there are still two issues that are in the development process.
One of China's chrome plate manufacturing industry is almost blank.
Since the reform and opening up, China's iron and steel industry has developed rapidly. At the end of the last century, China has become the world's top four steel producer. In 2000, the steel output reached 127 million tons and steel was 130 million tons. However, according to its variety and quality, it has a big difference from the developed countries in the world. About 40% of the products can meet the variety and quality needs, 30% of the varieties are not enough, 20% of the varieties are not good quality, and 10% of the varieties cannot be produced. In fact, China's steel products mainly consist of wire rods and profiles. Plates and pipes are relatively scarce. However, 25% of steel manufacturers are small enterprises, and 33% of steel products are small enterprises. They are old and outdated, and raw materials are unstable. Large enterprises also face the period of institutional reform and technological renovation. Therefore, as a whole, the gap in the quality of steel products in China is still large, the chemical composition fluctuates, the performance is unstable, the finished product has poor dimensional accuracy, and there are many surface defects. It is clear that there is a high scrap rate for users. Many companies have imported advanced equipment from abroad, but they still need to import materials because domestic raw materials are not applicable. From the perspective of sheet steel materials, although it has grown rapidly every year, domestically produced 19,031,100 tons in 2000, an increase of 10.20%, but the apparent consumption amounted to 39.23 million tons.
Therefore, the thin steel sheet has a very large gap in China. It is conceivable that in this context, the packaging industry wants to use chrome-plated steel plates and can only rely on imports. It is worth mentioning that at present, China's tin plate production capacity is surplus, but unfortunately it is mostly low block production line, the product can only be used for dry food packaging. High-grade tinplates that can meet the canning and beverage packaging still rely on imports. It can be seen from this that the raw materials themselves are restricting the development and progress of food packaging. This involves many of the scientific management we are advancing and coordinating development issues.
Second, how can our food canners know their packaging containers?
Currently, canned food processing companies mainly use tinplates. Their food processing, canning sales, and corresponding equipment and mechanisms are all based on canned tinplates. Any technological innovations and innovations are related to the benefits of the company. It is related to the credibility of the market. Therefore, the application of chrome plate materials is a risk to them. There must be scientific and economic analysis, reliable conditional guarantees, and entrepreneurs with a spirit of reform. Foreign countries have ready experience and equipment, and whether they can adapt to the current situation in China must be carefully explored.
It is understood that due to the limitations of the enterprise mechanism, the factory director dared not take the risk because it was a major transformation. It can thus be seen that there are conditional issues in the acceptance of new things by Chinese enterprises, and there are also ideological and cognitive issues. The key is still to establish a mechanism for continuous exploration and continuous improvement by enterprises. In particular, after China's entry into the WTO, it faces internationalization of the market. This must be strengthened without delay.
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