In the iron and steel materials and nonferrous alloy and polymer materials in the competition to continue to play its cheap and mature technology advantages, with high strength and effective strengthening measures can give full play to its potential strength, so far is still the most widely used in automobile manufacturing materials.
High strength steel plate
25% of the car's weight is in the body, and the lightweight parts of the body matter. In 1990s, 35 major iron and steel enterprises worldwide completed the task of "ULSAB Ultra Light Steel Auto Body". The research result shows that the high strength steel plate body 90% is now a large number of production (including high strength, super high strength steel plate and sandwich, weight loss) can not increase the cost under the premise of achieving weight reduction of 25% body (with 4 door sedan for reference), and the static torsional stiffness is increased by 80%. The static bending stiffness is increased by 52%, the first body structure modulus increase 58%, meet all crash regulations. Of course, this is the result of a study, the actual application of high-strength steel on the body has not yet reached such a high level. Based on the ordinary IF steel have developed high strength IF steel sheet and bake hardening of IF steel, while maintaining high formability and improves the strength and anti depression, as the body sheet thinning and realize the light to create the conditions.
The precipitation strengthening of elements such as Ti, Nb and V is added. The tensile strength of the steel plate is 500~750MPa, which can be used for wheels and other chassis parts.
The recent development of multiphase steels has considerable potential applications. The strength grade of ferritic bainitic steel is 500MPa, and the strength grade of dual phase (DP) steel and phase change induced plasticity (TRIP) steel is 600~800MPa, and the strength of composite (CP) steel is 1000MPa or higher. The steel is also well formed.
Laser tailor welded blanks (Tailored, Blank) are newly developed and applied lightweight steel plate technology. In the aforementioned ULSAB body, there are 18 parts using this technology.
Structural steel
Although the amount of steel material has been decreasing year by year, the dosage of high strength steel has increased considerably. High strength structural steel makes parts more compact and compact and helps lightweight cars.
(1) spring;
The most effective method of lightweight suspension spring is to increase the allowable design stress of spring. But in order to realize the lightweight under high stress, the high strength of materials is indispensable. Based on the traditional Si-Mn spring steel by reducing C and Ni added, Cr, Mo and other alloying elements such as V, the development of steel strength and toughness are very high, the allowable design stress of 1270MPa, the application of lightweight spring steel can achieve 40%. In traditional Cr-V spring steel, the addition of Nb can improve the delayed fracture resistance of the steel. Combined with the improved austenite rolling forming, the tensile strength of the steel can reach the level of 1800MPa.
Adding V to the Si-Cr steel used for the valve spring ensures the toughness by grain refinement and increases the strength by increasing C. After this improvement, the high cycle fatigue strength of the spring is increased by about 8%, and the light weight of the 15% can be realized. Through the finite element analysis, the spring section spring steel wire, which is evenly distributed inside and outside of the spiral spring, is developed to make the spring realize the light weight of 7%.
The effective way to increase the fatigue strength of spring is to shoot and process the spring. Shot peening of spring, besides the traditional stress shot peening, has developed two-stage shot peening. Shot peening and nitriding can also be used in combination.
(2) gear
The car engine has high power tendency, and has a tendency to compact the driver. This will increase the transmission gear load, so that the requirements of the bending fatigue strength and contact fatigue strength of gear steel are also increased accordingly.
To improve the content of Ni, Cr, Mo and other alloying elements in steel can improve gear steel hardenability and strength, but simply rely on alloy elements to strengthen gear steel will enable the cutting performance of the steel, the deterioration of heat treatment process is complicated, the cost of raw materials and the production cost will be greatly improved. In gear carburizing, in order to prevent or reduce the occurrence of abnormal layer, the content of Si and P in steel is reduced, and the amount of Mo is increased to 0.35%~0.45%, and the modified carbonitriding process is adopted. The improved steel can increase the impact life of gear material by 3~5 times, and if the above surface abnormal layer steel is added with powerful shot peening, the fatigue limit of gear can be increased by 20%~30%.
The non-metallic inclusion in gear steel is the starting point of fatigue crack and will reduce the strengthening effect of shot peening. Therefore, high purity gear steel has been developed. For example, SCM420HZ steel, the oxygen concentration is below 9ppm, the phosphorus concentration dropped to below 90ppm, compared with the low Si high Mo steel abnormal surface layer, gear tooth root bending fatigue life of 10%~17%, contact fatigue life increased by 25%.
High strength cast iron
Cast iron, due to its advantages in performance and cost, still occupies a place in automotive materials. The progress of cast iron makes it a new bright spot in automotive applications.
(1) nodular cast iron
Ferritic Ductile Iron, tensile strength of up to 500MPa, toughness is also higher, so more used for chassis parts, and some models even used as steering knuckle and other security.
Pearlite Nodular Cast iron is more powerful and can substitute for forged parts on some parts. The crankshaft of 4 cylinder car engine with balance block is strengthened by nodular cast iron and fillet rolling. It has become the standard process of automobile manufacturers in the United States, Germany, France and other countries. Because the density of nodular iron is about 10% smaller than that of steel, ductile iron can produce some light weight effect.
ADI (ADI-Austempered Ductile Iron) has very high strength and toughness, ductile iron grade manufactured by American and German standard, the highest strength reached 1400MPa, more than quenched and tempered steel and carburized steel strength level. Instead of steel, ADI can be used to manufacture automotive wheels, all wheel drive double linkages, steering knuckle arms, engine timing gears, crankshaft and connecting rod, etc.. By physical measurement, instead of forged steel, the crankshaft can be reduced by 10%, instead of aluminum alloy, the truck wheels can be reduced by weight of 0.5kg each.
(2) vermicular cast iron
Vermicular cast iron (Vermicular graphite cast iron) is also called the compacted graphite cast iron (Compacted graphite cast iron), the mechanical and physical properties and casting properties between the gray cast iron and ductile iron, it is suitable for high requirement of manufacturing strength and subjected to thermal cycling load parts, such as cylinder, cylinder head, exhaust manifold and brake drum etc..
The discovery of vermicular graphite cast iron is in the same time as nodular iron. However, the application of Sinter Cast process control system has wide prospects for the application of vermicular graphite iron because of the difficult control of vermicular process. Compared with the grey cast iron cylinder block, the weight of vermicular iron cylinder block is 16%, while the structural rigidity is increased by 12%~25%. The cylinder block made of vermicular iron can improve the friction and wear properties, reduce vibration and noise, and improve emission.
Powder metallurgy material
Near net shape characteristics of powder metallurgy components freedom and powder sintering technology and its application in automobile is increasing trend, especially the iron-based powder sintered material is applied in more and more complicated structure requires high strength.
The assembled powder metallurgy hollow camshaft is a new product in recent years. It is made of iron based powder metallurgy material, and then it is fixed on the hollow steel tube by sintering or mechanical method. Compared with conventional wrought iron or iron castings, it reduces weight by 25%~30%. This kind of camshaft has been used on high speed gasoline engine. With the increasingly severe working condition of diesel camshaft, the powder metallurgy hollow camshaft has the tendency to push the diesel engine.
The powder forging connecting rod has been successfully applied. In recent years, the production cost of a sintered powder metallurgy connecting rod technology has been low, and the lightweight of 11% can be realized.
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