In recent years, as the requirements of the automobile manufacturers have to reduce the cost of parts, lightweight and high strength, etc.. In particular, in overseas to obtain a more competitive price advantage than the new country, we must reduce costs, production in the local competitive car. At the same time, in order to deal with environmental issues, must reduce CO2 emissions, to reduce fuel consumption, we must try to reduce the weight of the body.
To reduce the cost of parts, of course, also includes the need to reduce the cost of bolts. In the production cost of the bolt, the stock cost is the main part, so the most effective way is to reduce the cost of the material itself. Japanese auto manufacturers to reduce the cost of material supply, are studying the use of overseas cheap materials. But because some of bolt wire clean degree of quality demand is higher than that of the plate or bar and can not fully meet the Japanese auto industry requirements of quality. Therefore, the use overseas steel car manufacturers are few.
On the other hand, in the light of quantification, the automobile manufacturers have carried out a variety of research. In order to meet the above requirements of the users, the bolt manufacturers have also carried out a variety of research to make the bolt lightweight. As a measure of the weight of the bolt, some light metals such as aluminum or titanium are used, some of which reduce the size of the bolt. In these various measures, although the aluminum is much lighter than the iron, but the strength is low. Although titanium is lighter than iron, it has the same strength as iron, but it is not easy to process, and the material cost is very high. Based on the above reasons, the automobile bolts in light weight to achieve the actual results of the example is very few.
While reducing the size of the bolt can reduce weight, but if the same strength grade to reduce the size of the bolt, due to the reduction of the area of the bolt, the bolt tightening force will drop. Therefore, in order to ensure the same degree of strength, but also to reduce the size of the bolt, we must increase the strength of the bolt.
In the current JIS and other public standards and car manufacturers own standards, only the strength of the following 12.9 bolts to make provisions. When the strength is more than 12.9, the fracture characteristics of the ordinary quenched and tempered bolts will become worse. In some 10.9 and 12.9 high strength bolts has been used, if want to achieve the lightweight by reducing the size, we must use intensity more than 12.9 bolts, and use some methods to improve the delay fracture characteristics.
Quenched and tempered bolts to achieve high strength, and some by adding alloy elements to improve the delay fracture characteristics. Although the addition of alloying elements can improve the delay fracture characteristics, but because of the high cost of alloying elements, the cost of the rise is inevitable.
Another measure is the use of non quenched and tempered bolts. The non quenched and tempered bolts are not required to be quenched and tempered after forming, mainly through the processing of the material to ensure the strength. The metal structure of the non quenched and tempered bolt is completely different from that of the quenched and tempered bolt. The use of non quenched and tempered bolt is the carbon content is higher than that of quenched and tempered bolt material, the billet stage of controlled cooling and heat treatment, can obtain high section shrinkage rate, by drawing processing formation hardening, bolt forming stage in further work hardening can ensure the bolt strength. Ordinary bolt after forming although no quenching and tempering, but in order to pass cold forging to eliminate internal bolt strain. Therefore, it is necessary to bluing treatment. The disadvantage is that the hardness of the material is higher than that of the quenched and tempered, and it is very difficult to form.
At present, the strength of the non quenched and tempered bolt used for the engine is 1600MPa, and the strength of the non quenched and tempered bolts used in the body has reached 1400MPa. It can be expected that in the future the demand for such high strength bolts will be further expanded.