In the sheet metal industry, stainless plates are more and more used. Stainless steel products with its exquisite appearance, excellent corrosion resistance, high temperature oxidation resistance and high and low temperature strength and quite people's favor, more and more widely used in decoration, light industry, civil hardware, kitchen equipment and appliances and other industries. Due to this kind of product appearance quality request is higher, in the whole process of product, to ensure the high brightness of the surface of the product is not really challenging cuts and bruises, especially due to the stainless steel sheet deep drawing properties of mold material selection, heat treatment, machining and processing lubrication problems directly affect the product cost and mold gold refining, production and life.
In the sheet metal industry, stainless plates are more and more used. Stainless steel products with its exquisite appearance, excellent corrosion resistance, high temperature oxidation resistance and high and low temperature strength and quite people's favor, more and more widely used in decoration, light industry, civil hardware, kitchen equipment and appliances and other industries. Because this kind of product appearance quality is higher, in the whole process of Progressive Die Stamping products, to ensure the high brightness of the surface of the product is not really challenging cuts and bruises, especially due to the stainless steel sheet deep drawing properties of mold material selection, heat treatment, machining and processing lubrication problems directly affect the product cost and mold gold refining, production and life.
Drawing characteristics and bonding nodulation of stainless steel sheet
Due to the high yield point of stainless steel, high hardness, cold hardening effect is significant, stainless steel sheet deep drawing its characteristics are as follows:
1) Due to the poor thermal conductivity than ordinary low carbon steel, the required deformation force is large;
2) when stainless steel sheet is drawn, the plastic deformation is violently hardened, and the sheet is easy to wrinkle when drawn, which requires larger blank holder force;
3) the springback caused by the bending and reverse bending of the sheet material at the rounded corner of the deep drawing die usually forms a depression deformation on the side wall of the product, which makes the products with higher dimensional accuracy and shape requirements need to increase the plastic process to achieve;
4) Bonding nodulation is easy to occur in the process of drawing stainless steel sheets.
From the microscopic point of view, the sheet metal and die surface are uneven and rough surface. Due to the large blank holder force in the process of deep drawing, the load is borne by the local convex part of the unit pressure is very large; and because of the relative motion between the sheet and the die and the plastic deformation of the sheet produces heat energy, the viscosity of the lubrication film decreases, the strength decreases; a convex on the sheet metal under high pressure, the instantaneous high temperature, the movement under the action of shear, lubricating membrane rupture, sheet metal, direct contact with the mould convex parts by a convex mold pressing into pieces, and push ahead into a convex mold, such as the temperature is high enough, allow debris to soften, melting, dry welding on the mould, to form tumors. Once formed, it is difficult to fall off the bond tumor, and the more sticky the greater, resulting in serious scratches on the surface of stainless steel sheet drawing products. In addition, the drawing speed and the deformation of sheet material also play an important role in the formation of bond tumor. How to avoid the formation of bonding noodles in drawing die and improve the surface quality of drawing parts are the technical difficulties in drawing stainless steel sheets.
The measures
In the process of deep drawing of stainless steel sheets, the problem of bonding noodles has been troubling the production site, which brings great trouble to the producers. However, because the formation of bonding nodular involves tribology and other problems, there are many influencing factors. At present, we can only put forward measures from different angles to prevent the formation and reduction of adhesive tumors.
Material selection and heat treatment of mold working part
In view of the bond tumor problem, the choice of mold materials should be based on the affinity relationship between stainless steel sheet and mold materials to pay attention to two points: the choice of one is strong adhesion resistance, the second is wear resistant and friction reduction of mold materials. Generally speaking, metal lattice type, lattice spacing, electron density, electrochemical properties of the same metal, their mutual attraction, dissolution ability are strong, easy to adhere to each other, resulting in higher friction coefficient. Cr, Ni and Fe are highly soluble, so it is more likely to occur when drawing with steel die. The practice shows that casting aluminum bronze and duralumin bronze have a better anti-sticking effect. Using tungsten carbide steel bonded hard alloy manufacturing die compared with Cr12Mov soft nitride manufacturing die life increased several times, and non-stick die; if the code 3054 alloy cast iron is used, only flame quenching is carried out on the surface of the mold, and there will be no bonding tumor on the surface of the mold. In addition, hard alloy inserts can be used in the vulnerable parts of the mold, which have excellent compressive performance, super wear resistance and durable surface roughness and dimensional precision control. But because of the price problem, it is used less in production.
Surface finishing of the working part of the die
The surface quality of stainless steel drawing die is very high. The lower surface roughness can play a role in reducing friction and improving adhesion resistance. After grinding the drawing die, it is more important to eliminate machining traces. However, grinding and polishing processes are often neglected in mold manufacturing. It should be pointed out. In the whole process of mold processing, polishing work should account for one third, because the appearance quality of stainless steel products largely depends on the polishing technology of the mold. The surface roughness of the die is reduced, the grinding times of the die are reduced, and the service life of the die is improved accordingly. If the die surface polishing is not enough, coupled with the stainless steel sheet drawing, it is easy to cause the characteristics of bonding tumor, so the product from deep drawing is seriously scratched. And this scratches on the product with polishing treatment to solve both time-consuming and ineffective. Therefore, we should make great efforts in the mold polishing process. Only by improving the surface processing accuracy of the mold can the scratches of the product be reduced, and the mold repair life can be greatly improved.
In recent years, a variety of new polishing technologies and polishing tools have appeared in China. Developed ultrasonic, electrolytic polishing, abrasive jet, extrusion honing and other new technology, new equipment. According to the data, the electrochemical polishing test of CrWMo, 3Cr12W8V and Cr12 molds shows that the surface roughness of the mold cavity can be reduced from the original RA3.2-RA1.6 to RA0.4-Rao. 2 in only 5-10 minutes. At the same time, the surface hardness can be improved by electrochemical polishing to improve the wear resistance. For example, ultrasonic polishing machine can be used for the soft nitride treatment of the cavity fine polishing, it can avoid the shortcomings of manual polishing easy to destroy the nitride film. We should actively apply and summarize the new polishing technology.
Processing lubrication
According to the characteristics of stainless steel drawing, the formation of bonding nodus is due to the direct contact between the sheet metal and the die, which is a theoretical indisputable fact, so the choice of lubricant or coating agent is the first point in the process of sheet metal drawing forming lubrication film from beginning to end does not break and play a lubricating role. "Anti - adhesion and friction reduction" is the basic starting point of lubricant selection.
Generally, adding a certain proportion of extreme pressure additives or using solid lubricants can achieve better results. This is mainly to improve the lubrication ability of lubricant on the metal surface, to produce sulfur, phosphorus, chlorine compounds at high temperature and metal surface chemical reaction, the generation of iron sulfide, ferric chloride, etc., to strengthen the strength of the oil film and enhance the adsorption capacity, better lubrication mold and product surface. Solid lubricant is filled into a small pit on the metal surface, so that the dry friction contact point is reduced to a minimum. In addition, solid lubricant has high stability, can also play a lubricating role at high temperatures, and is not easy to occur mold bonding. Usually in production according to the degree of deformation and the actual situation of the product selection and formulation (the relevant stamping manual can be found in the formulation).
In addition, fat, mineral oil, synthetic grease, grease, soap can also be composed of lubricant, and the effect of stainless steel shallow deep drawing is good. Organic mineral oil containing water-soluble emulsion or diluted with oil can also be used for shallow drawing. In the lubricant, adding graphite can play the role of anti adhesion, but graphite after the cleaning is more difficult. If the code 3054 alloy cast iron is used to make the mold, then the general lubricant is better.
According to the data, a layer of metal soft die (such as copper, zinc, lead, etc.) can be obtained on the surface of sheet metal after salt bath treatment, and the phenomenon of sticking die will not appear in the process of deep drawing. In addition, in recent years, a kind of organic polymer lubrication film with polyvinyl butyl as the main body has been developed. After processing, the surface of the blank gets a layer of organic lubrication film, which can be deformed with the sheet material, which not only avoids the direct contact between the mold and the sheet material, prevents the adhesion of the mold, and ensures the surface quality of the product. Moreover, the friction effect between die and sheet material is greatly weakened, and the lubrication effect is good. The experiment shows that the effect is good.