In the past, the engine connecting rod was made of medium carbon steel or medium carbon alloy steel after quenching and high temperature tempering. The hardness after treatment was generally 228-269hbs. However, there are some defects such as difficult machining, poor surface quality, easy crack in quenching, and high rejection rate. In order to save energy, reduce production cost, improve product quality and workpiece machining efficiency, non quenched and tempered steel (microalloyed steel) was developed and used in production, that is, a small amount of vanadium was added to the alloy steel, which was air cooled after forging, and the same mechanical properties as those after quenching and high temperature tempering were obtained through precipitation strengthening. Quenching and high temperature tempering were omitted, thus reducing production cost. At present, the connecting rod of ERW EQ6100 engine is made of f35mnvn steel instead of 40MnB quenched and tempered steel. By 1998, 400000 vehicles had been loaded. In order to reduce the amount of machining and reduce the cost, a powder metallurgy method has been developed to produce automobile connecting rods. The “near net shape” parts are formed by powder pressing and sintering, which greatly reduces the amount of cutting. Moreover, the quality tolerance of powder metallurgy parts is small, which is more suitable for V6 / V8 engines with strict quality control. The powder metallurgy method is widely used to produce automobile connecting rods in the United States.
In the mid-1990s, the steel plant cooperated with the automobile plant to develop the steel for cracking automobile connecting rod (it is actually a high carbon steel with a carbon content of about 0.7%). Compared with non quenched and tempered steel and powder metallurgy, this kind of steel has advantages in cost and service performance. After forging, air cooling does not need heat treatment. After cracking, the contact surface of automobile connecting rod and connecting rod cap does not need machining, which saves processing costs. After assembly, the cracking surface of connecting rod and connecting rod cap is in close contact and locked with each other to prevent their mutual movement, which improves the stiffness of crankshaft parts and improves engine performance, This steel is widely used in Europe to produce automobile connecting rods. Germany began to use this kind of steel in large quantities in 1955. 19 engines use this kind of steel to produce automobile connecting rods. So far, 60 different engines use this kind of automobile connecting rod; In 1997, 6million automobile connecting rods made of this steel were forged, and the size is not limited. Different types of cracked connecting rod steels are used from motorcycle connecting rods to heavy-duty truck connecting rods. The connecting rod of Jetta car of FAW Volkswagen Co., Ltd. is also made of this kind of steel. The connecting rod of 6DL series diesel engine developed by FAW now adopts the cracking process. The material is high carbon non quenched and tempered steel FAS70S2, which is currently imported from abroad. The localization test of FAS70S2 non quenched and tempered steel has been carried out in 2005. The main technical characteristics of the steel are narrow range of chemical composition, high requirements for steel surface quality, and there are still some problems in domestic steel production.
| Steel grade | Treatment process | Average breaking load /kn | Minimum sectional area /mm2 | Overall tensile strength /MPa |
| S53C | Conditioning | 221 | 257 | 976 |
| YF35MnV | Controlled cooling after forging | 230 | 257 | 1021 |
| YF40MnV | Controlled cooling after forging | 242 | 257 | 1102 |
| 35MnVNbS | Controlled cooling after forging | 286 | 257 | 1167 |
Non quenched and tempered steel has been widely used in foreign automobile industry because of its series of advantages. Germany, Sweden and Japan have better research and application of non quenched and tempered steel. In Volkswagen, the annual output of car connecting rods made of 27MnSiVS6 non quenched and tempered steel reaches 2.5 million pieces; Volvo of Sweden consumes about 25000t of steel every year to manufacture automobile parts. Ford of the United States, Fiat of Italy and Volga of Russia all use non quenched and tempered steel to manufacture their crankshaft, connecting rod and other parts. In the development and promotion of new varieties and technologies, Japan has taken the lead in the world in recent years. In Japan, 75% of connecting rod non quenched and tempered steel was produced in 1984.