Through the analysis of the problems in the use of metal sheet drawing die materials, the reasons for the defects such as scratches and scratches on the surface of the workpiece are found out. The characteristics of silicon nitride ceramic materials are introduced, and the use of silicon nitride ceramic materials to manufacture metal sheet drawing concave molds is proposed to ensure the surface quality of the drawn parts.
The protective casing is one of the parts of an environmental protection equipment. The material is stainless steel SUS304L, and the production batch is large. After assembly, the parts fall on the equipment, so the surface quality requirements are relatively strict. After stretching, the surface of the parts is required to be scratched and scratched not more than 0.10mm. The forming of this product adopts blanking and 3 times of stretching process.
2.1 Requirements for metal sheet drawing on mold materials
- (1) High hardness and good wear resistance.
- (2) Good anti-adhesion.
- (3) Good heat treatment performance (hardenability, hardenability, micro-deformation, etc.).
- (4) Certain toughness.
- (5) Good machinability.
2.2 Selection principle of ferrous metal drawing for mold material
Selection principle of mold material:
Molds of different materials have different heat treatment, different surface states, affinity with the material to be processed, and different degrees of high temperature friction bonding. Therefore, it is necessary to select the mold material appropriately and economically according to the properties of the drawn material, surface quality requirements, production batch and other factors. Usually, when drawing hard materials, soft mold materials are used, and when processing soft materials, hard mold materials should be used.
2.3 Selection of protective shell drawing die material
Cold work die steel (such as Cr12MoV, Cr12, etc.) is selected as the material of each drawing die, and nitriding treatment is carried out after heat treatment. In actual production, when about 20 pieces were stretched for the first time, there were obvious scratches on the surface of the parts, and the mold had to be polished. Due to frequent polishing of the concave mold, the size of the workpiece is out of tolerance, resulting in the scrap of the mold, which once made the production impossible. Due to the large batch of the parts, in order to ensure the mold has a high service life, and to avoid defects such as scratches and scratches on the surface of the blank parts, each drawing die is made of cemented carbide. After using the cemented carbide composite die, although the service life of each drawing die is improved to a certain extent, the surface of the blank part often has defects such as scratches and scratches, and the die needs to be polished. Due to the high hardness of cemented carbide materials, polishing is more difficult.
3 Causes and solutions for scratches on the mold
3.1 Causes of scratches on the mold
During the stretch forming process of the stainless steel sheet, the material has to generate a great resistance during the deformation and flow process. Even if the surface roughness of the mold is very low, frictional force must be generated between the workpiece and the mold. Since the stainless steel material and the mold material have a certain adhesion, the mutual friction between the workpiece and the mold surface is aggravated. In addition, during the stretch forming process of the stainless steel sheet, due to the dislocation and distortion of the internal metal lattice, and the affinity between stainless steel and the metal mold, a large amount of heat energy will be generated, and the contact part between the workpiece and the mold will appear due to the instantaneous high temperature. Cold welding bonding phenomenon, high temperature cold welding bonding, resulting in serious scratches on the surface of the workpiece, the quality cannot be guaranteed, although the hardness of cemented carbide is high, the mold needs to be polished frequently.
(1) Characteristics of silicon nitride (Si3N4) ceramic materials.
Silicon nitride (Si3N4) ceramics are formed by molding silicon powder on a hydraulic press, and then sintered in an electric furnace after rough machining. After sintering, it is air-cooled, ground to obtain the final size, and finally polished.
Reaction sintered silicon nitride ceramic materials have the following characteristics:
- a. High wear resistance. The silicon nitride ceramic material has high wear resistance, which can ensure that the drawing die can work for a long time within the allowable deformation degree of the workpiece, and the size of the drawing piece will remain unchanged.
- b. Excellent polishing performance. Silicon nitride ceramic materials have good polishing properties, and can be processed into mirror-like die holes to ensure the high quality of the surface of the drawn metal parts.
- c. Low adhesion to stretched metals. Silicon nitride ceramic are inorganic non-metallic materials and have no affinity with metal materials, which avoids scratches on the surface of the workpiece caused by adhesive wear.
- d. Silicon nitride material has high hardness. The hardness can reach above 2,000HV.
- e. The friction coefficient is small. When the surface of the mold is finished to 0.005~0.025mm, the friction coefficient is as small as 0.01, the friction coefficient is small, and the energy consumption is reduced by more than 60% compared with the steel mold.
- f. Good corrosion resistance. Silicon nitride ceramic materials are inorganic non-metallic materials with good corrosion resistance to cooling lubricants.
- g. The thermal expansion coefficient is close to that of steel.
- h. Although the compressive strength of silicon nitride ceramic material is high, it is brittle and easy to break. In order to prevent the silicon nitride ceramic mold from expanding and cracking during stretching, a prestressed mold sleeve is added to the ceramic mold. In order to ensure that the ceramic mold has a high life.
(2) Silicon nitride ceramic materials have wide applicability.
Due to the above characteristics of silicon nitride ceramic materials, silicon nitride ceramic die materials can stretch various grades of ferrous and non-ferrous metal materials, especially for stainless steel materials.
(3) Requirements of silicon nitride ceramic die for drawing equipment.
In view of the high brittleness of silicon nitride ceramics and the inability to withstand large impact forces, when using silicon nitride ceramic die for drawing production, hydraulic presses or double-action drawing presses with stable pressure and low impact force are generally used.
4 The material of the drawing die is changed to silicon nitride ceramics
Due to the above advantages of silicon nitride ceramic machining materials, silicon nitride ceramic materials are selected to manufacture the composite die for each drawing of the protective casing. The designed die for the third stretch of silicon nitride ceramic composite die
It avoids the phenomenon of cold welding and bonding between the metal material and the silicon nitride ceramic die due to the instantaneous high temperature, and the surface quality of the workpiece is free from defects such as scratches and scratches. At the same time, the friction coefficient is small, which reduces the tensile force, reduces the tonnage of the stretching equipment, and saves energy. The service life of the silicon nitride ceramic composite die is as high as that of the cemented carbide composite die.
Through practical application in recent years, silicon nitride ceramic materials are used in the drawing and forming of stainless steel sheets, which significantly improves the appearance quality of the workpiece, and the service life of the mold is significantly improved. The production cost is reduced, and better economic and social benefits are achieved.