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Bolts and screws are the most basic mechanical components, which are widely used in various industries such as automobiles, industrial machinery and construction. Unlike welding and riveting, the biggest feature of bolts is that they can be simply removed even after fastening and can be reused by re-fastening.

1 Production process
The material of the bolt is generally coiled wire. After secondary processing such as pickling, lubrication, annealing and wire drawing, the cold forging method is used to form the screw head and thread of the bolt, heat treatment and surface treatment. Due to the different types of products, some need to be heat treated before forming the thread, and some do not need to be heat treated or surface treated. Secondary processing is generally handled by professional processing personnel, but sometimes it is handled in the bolt production plant.
Shaping of 1.1 screw head
Generally, the process of cold forging the bolt head by using a forging machine such as a top upsetting machine or a push rod machine is called "top upsetting". The so-called cold forging, is processed at room temperature, it is with the blank after heating temperature processing and hot processing relative to the said method.
The process of forging is to cut the coiled steel into the appropriate length and use several forging die devices to forge the material. Forging includes upsetting, deep drawing, reverse extrusion, trimming and other 4 processes. Upsetting forging is the processing of squeezing the material from one end to make it expand to a larger diameter than the original diameter. Deep drawing, on the contrary, is to squeeze the material from one end to make it smaller than the original diameter. Reverse extrusion is the processing of extruding from the end face of the material to a forging die smaller than the diameter of the material, so that the material is extruded outward while perforating; dressing is the process of removing excess thickness with a forging die. Depending on the shape of the processed product, these processing methods may be used separately or in combination for forming. The more complex the shape of the product, the more need to increase the processing process, slowly forming processing, but ordinary bolts only 2-5 processing processes can be processed.
Forming of 1.2 Thread Part
In the thread forming process of the bolt, the thread is cold-forged by using a roll forming machine, and the forming method is the same as the forming method of the bolt head.
The processing of the thread shape is to place the blank between 2 1 sets of forging dies for clamping, one side rotates one of the two forging dies, and the other side rotates the blank to become the thread shape through plastic processing. A forging die for thread forming is called a "roll forming die".
There are three types of roll forming machines: flat forging die roll forming machine, round forging die roll forming machine and planetary roll forming machine. The flat forging die rolling forming machine is equipped with two ordinary rolling forming forging dies, which fix one side and let the other side move back and forth to roll the blank. The round forging die roll forming machine rotates the two cylindrical roll forming forging dies installed in parallel in the same direction to roll the blank sandwiched between the two forging dies. The processing method of the planetary roll forming machine is to clamp the blank between the cylindrical forging die and the fan-shaped forging die, and roll forming is carried out by rotating the cylindrical forging die.
1.3 heat treatment
Cold forging bolts are generally made of materials with hardness suitable for plastic processing. Materials with high carbon content or materials with alloying elements are not easy to process due to their hard materials. Therefore, some materials need to be annealed to soften the materials. Most materials do not meet the required strength in the case of cold forging. Heat treatment is the treatment after cold forging, it can make the bolt has the required strength and mechanical properties, is the most important process in the bolt production process.
Bolts can be divided into various strength grades according to the location and purpose of use. In order to make the formed bolts have the required mechanical properties, heat treatment is required.
1.4 surface treatment
Bolts used in automobile engines often adhere to lubricating oil, and therefore do not rust even without surface treatment. However, this kind of bolt is only a small part, most of the bolts are used in a corrosive environment, so if the surface treatment is not carried out, it will quickly rust. If rusty bolts are placed here, they will not be able to be screwed open after corrosion. In severe cases, bolts will break and cause major accidents. Therefore, a bolt used in a corrosive environment needs to be subjected to a surface treatment such as plating.
The surface treatment of bolts can be roughly divided into two types: electroplating and coating. The most widely used is electroplating, which has the advantages of low cost and good corrosion resistance. When the corrosion resistance is required to be higher than that of electroplating, alloy electroplating such as zinc-plated iron and zinc-plated nickel can be used, or a zinc-aluminum composite film can be coated.
2 Development Trend of Automobile Bolt
In recent years, automobile manufacturers have demanded to reduce the cost of parts, lightweight and high strength. At the same time, in order to deal with environmental problems, CO2 emissions must be reduced, and in order to reduce fuel consumption, the body weight must be reduced as much as possible.
In the production cost of the bolt, the blank cost accounts for the main part, so the most effective way is to reduce the cost of the material itself. In order to reduce the supply cost of materials, Japanese automobile manufacturers are studying the use of cheap overseas materials.
In order to meet the requirements of users, bolt manufacturers have also carried out a variety of research to make bolts lightweight. As a measure for reducing the weight of the bolt, some use a light metal such as aluminum or titanium, and some reduce the size of the bolt.
Reducing the size of the bolt can reduce the weight, but if the size of the bolt of the same strength grade is reduced, the tightening force of the bolt will decrease due to the reduction of the cross-sectional area of the bolt. Therefore, in order to ensure the same degree of strong force and reduce the size of the bolt, it is necessary to increase the strength of the bolt.
In the current JIS and other public standards and automobile manufacturers own standards, only the strength of the bolts below 12.9 to make provisions. When the strength grade of ordinary quenched and tempered bolts exceeds 12.9, the delayed fracture characteristics will deteriorate at once. In some places where 10.9 or 12.9 high-strength grade bolts have been used, if you want to achieve lightweight by reducing the size, you must use bolts with a strength grade of more than 12.9, and use some methods to improve the delayed fracture characteristics.
In order to achieve high strength, some of the tempered bolts improve the delayed fracture characteristics by adding alloying elements. Although the addition of alloying elements can somewhat improve the delayed fracture characteristics, an increase in cost is inevitable due to the high price of alloying elements.
Another measure is the use of non-quenched and tempered bolts. Non-quenched and tempered bolts do not need to be quenched and tempered (heat treatment) after forming, and the strength is mainly ensured by the work hardening of the material. The metal structure of non-quenched and tempered bolts is completely different from that of quenched and tempered bolts, and it has strong resistance to delayed fracture. Non-quenched and tempered bolts use materials with higher carbon content than quenched and tempered bolts. Through controlled cooling and heat treatment at the blank stage, high section shrinkage can be obtained. After wire drawing is formed by work hardening, further work hardening at the bolt forming stage can ensure the strength of the bolt. Although ordinary bolts are not quenched and tempered after forming, they must be treated with bluing in order to eliminate the strain inside the bolts by cold forging. The disadvantage is that the hardness of the material is higher than that of the quenched and tempered type, which is very difficult to form.
At present, the strength of non-tempered bolts for engines is 1600MPa, and the strength of non-tempered bolts for car bodies has reached 1400MPa. It can be expected that the demand for such high-strength bolts will further expand in the future.