Since the first microalloyed non quenched and tempered steel was successfully developed in West Germany, non quenched and tempered steel has been widely used all over the world. However, in the process of application, people feel that non quenched and tempered steel has more strength and less toughness. Therefore, improving the toughness of non quenched and tempered steel has become the main goal of researchers all over the world, and runs through the development process of microalloyed non quenched and tempered steel. For more than 30 years, researchers have greatly improved the toughness and strength of non quenched and tempered steel by changing the chemical composition, microstructure, strengthening mechanism and controlled rolling (forging) and controlled cooling process.
For the ferritic pearlitic non quenched and tempered steel used to manufacture automobile connecting rods, the following technologies and methods have been developed in recent years to improve the toughness of ferritic pearlitic non quenched and tempered steel.
- Reasonable use of strengthening elements to improve toughness. For example, by reducing C content, increasing Si and Mn content and adopting composite microalloying method, the toughness of steel can be improved without reducing the strength of steel.
- Grain refinement. Refining the grain size of steel can effectively improve the toughness of non quenched and tempered steel without reducing the original strength. A small amount of Ti and Al are added to the steel, and the austenite grain boundary is pinned by precipitation of tin and AlN, which prevents the growth of austenite grain during heating, inhibits the recrystallization of austenite grain during deformation, and refines austenite grain.
- Using oxide metallurgy technology. During the steel smelting process, the composition, quantity and distribution of oxide inclusions (Al2O3, MnO, FeO, (Ti, Mn) 2O3) in the steel are controlled, so that a large number of fine MNS particles are precipitated from the grains. During cooling, V and n elements are precipitated on the MNS particles in the form of VN, becoming ferrite nucleation positions, evenly distributed in the austenite grains, promoting the formation of intragranular ferrite (IGF) structure, and greatly improving the strength and toughness of the material.
Controlled rolling (forging) and controlled cooling technology is the main way to improve the comprehensive mechanical properties of non quenched and tempered steel. On the basis of properly adjusting the chemical composition of the steel, it can refine the structure and improve the strength and toughness of the steel by controlling the heating temperature, final rolling (forging) temperature, cooling speed after rolling (forging), deformation amount and deformation rate.
Increasing the heating temperature can make the carbon and nitride of V, Ti and Nb gradually dissolve into austenite, and a large amount of dissolved microalloyed carbon and nitride precipitate during cooling, which can improve the strength and hardness of steel; On the other hand, with the increase of temperature, the austenite grain grows up, which leads to the coarsening of microstructure and the decrease of toughness. Properly controlling the lower finishing rolling (forging) temperature can increase the degree of grain breakage, reduce the driving force of recrystallization and refine the grain, which is conducive to improving the toughness. Rapid cooling after rolling (forging), large transformation undercooling, increased nucleation rate of new phase, refined grain and effectively improved strength and toughness of steel; But too fast will also have adverse effects. For example, large internal stress will produce Martensite for bainitic steel, which will affect the toughness and cutting performance of non quenched and tempered steel. At the same deformation temperature, when the deformation variable and deformation rate increase, the austenite grain is obviously refined, the microstructure is fine and uniform after rolling (forging), and the strength and toughness have an increasing trend. In order to get the best strength toughness fit, the heating temperature, final forging temperature, cooling rate after forging, deformation and other factors should be combined for comprehensive control.
The thickness of automobile connecting rod is uneven and varies everywhere. Its hardness index shall be the same. The chemical composition, metallographic structure and relevant mechanical properties of domestic non quenched and tempered steel automobile connecting rod shall meet the technical indexes of similar German products. Compared with foreign similar materials, the domestic non quenched and tempered steel has the following differences: the quality of hot rolled products is poor; There is no certain control range for residual elements, resulting in great fluctuations in mechanical properties; The chemical range specified in the standard is wide. The domestic performance requirements for non quenched and tempered steel include: hardness and strength, plasticity and toughness, fatigue performance, machinability, etc. There are many factors that affect the properties of non quenched and tempered steel, the most important one is toughness. Improving and improving the toughness of non quenched and tempered steel can further expand its application range.
The advanced production process of using high carbon non quenched and tempered steel to adopt the controlled cooling mode after forging, and using the expansion and fracture method (i.e. separation method) to separate the big end of automobile connecting rod to replace the traditional quenched and tempered steel, and using quenching + high temperature tempering to produce car connecting rod is an advanced technology for the production of automobile parts abroad in the 1990s. The car engine connecting rod is made of non quenched and tempered steel instead of quenched and tempered steel, and the service performance of the car connecting rod can be achieved by using the forging waste heat to control the cooling rate, thus eliminating the traditional re quenched and tempered treatment after forging. The big end of the automobile connecting rod is separated by the advanced “expanding and breaking method”, which not only saves a lot of machining processes, but also has excellent coincidence between the big end of the automobile connecting rod and the big end. Therefore, using non quenched and tempered steel instead of quenched and tempered steel to manufacture mechanical parts can save energy, simplify the process flow, improve the cutting performance, greatly reduce the production cost and have higher economic benefits.
For the composition of cracked automobile connecting rod steel, it is necessary to ensure sufficient strength after forging and air cooling to meet the needs of automobile connecting rod service performance; Secondly, it is necessary to ensure that the automobile connecting rod has the minimum plastic deformation after cracking and the ovality of the crankshaft hole during assembly meets the specified requirements. Therefore, the carbon content of the steel should be increased from about 0.45% of the general medium carbon steel to about 0.7% of the eutectoid composition, so as to ensure that the metallographic structure after forging air cooling is all pearlite, so that the plastic deformation after cracking is the minimum; Finally, ensure that the steel has good machinability, which is not lower than the original medium carbon steel. After the carbon content is increased, the hardenability of the steel will be increased. If the manganese content remains unchanged, the hardness of the automobile connecting rod will be improved after forging and air cooling, and bainite may appear in the metallographic structure, deteriorating the machinability. Therefore, the manganese content should be appropriately reduced. However, in order to improve the machinability, the sulfur content must be properly increased. The combination of manganese and sulfur should not be too low, and the manganese content should be at least 3 times that of sulfur. Thus, the lower limit of manganese content is determined. Therefore, it is necessary to comprehensively consider the above.
In recent years, some engine manufacturers in China have introduced the expanding and breaking connecting rod processing technology. This technology is to separate the automobile connecting rod and cover with the expansion and fracture method. The expansion and fracture method not only solves the problem of roundness error in the assembly of automobile connecting rod, but also saves the processing process, which is progressiveness in technology. At present, some manufacturers in China, such as Chery Automobile Co., Ltd. and FAW Volkswagen Co., Ltd., have developed expansion type automobile connecting rod processing lines, which have been widely used in production and have a good development prospect. The expanding and breaking automobile connecting rod has high requirements for materials.
At present, the expanding type automobile connecting rod produced by Baicheng Zhongyi Precision Forging Co., Ltd. is made of materials imported from Germany. The material brand is C70S6, and the main components are: 0.67-0.73 C, 0.15-0.25 Si, 0.45-0.55 Mn, P ≤ 0.045, 0.06-0.07 s. In addition, there are trace alloy elements such as nickel, chromium, molybdenum and vanadium. The range of alloy elements is very strict. Refining outside electric furnace and smelting furnace, vacuum degassing and rolling φ 40mm round bar. The microstructure of hot rolled material is pearlite + a small amount of ferrite. Forgings shall be normalized by forging waste heat. The metallographic structure shall be pearlite + ferrite, and the ferrite content shall not exceed 10%. See table for mechanical property index requirements.
| Tensile strength MPa | Yield strength MPa | Elongation | Reduction of area |
| 900-1050 | ≥520 | ≥10% | 20%-40% |
Production process flow of automobile connecting rod: blanking – medium frequency induction heating – roll forging blank – trimming (thermal correction at the same time of trimming) – residual heat normalizing – strong shot blasting – CS inspection – flaw detection – fine pressing – surface inspection – weight inspection – packaging.
The residual heat normalizing of forgings shall be controlled by special air cooling equipment. The air cooling equipment is divided into three conveyor belts in zigzag structure. Each conveyor belt is equipped with a blowing device and an air-cooled motor. The cooling speed of automobile connecting rod forgings on each conveyor belt can be controlled by adjusting the speed of the air-cooled motor (strong, medium and weak) and the speed of the conveyor belt.