Application of different types of steel pipes in the automotive industry

In the modern automobile industry, steel pipes, as an indispensable basic material in automobile manufacturing, are widely used in many fields such as body structure, chassis and transmission system. As automobiles put forward higher and higher requirements for lightweight, high strength and corrosion resistance of materials, the application of various steel pipe technologies has become more and more mature, especially with the support of hot stamping and coating technology, the performance of steel pipes in the automobile industry has become more and more excellent.

The common types of steel pipes in the automobile manufacturing process mainly include seamless steel pipes, electric resistance welded pipes (ERW steel pipes), stainless steel pipes, high-strength steel pipes and galvanized steel pipes. Different types of steel pipes play their respective roles in different parts of the car.

Seamless steel pipes are the main materials for manufacturing automobile half-axle sleeves and drive axle housing shaft tubes. The cantilever frame tube is used for the rear axle of the chassis, and the steel pipes for the front axle assembly of the chassis include shaft sleeves, bearing sleeves, and brake pedal bearing tubes. The key types of seamless steel pipes include tubes for automobile mufflers, rotating shafts, thrust pipes, cooler pipes, exhaust pipes and their control shafts, shock absorber oil storage pipes, etc. Seamless steel pipes have excellent pressure resistance and fatigue resistance, and are usually used in key parts of automobiles such as drive shafts, steering shafts and transmission shafts. These parts need to withstand huge mechanical stress and friction during movement, so the high strength and durability of seamless steel pipes are extremely important.

ERW steel pipes are widely used in automobile exhaust systems, seat brackets and chassis structures due to their high production efficiency and good dimensional accuracy. These parts have high requirements on the weight of the material, and the lightweight advantage of ERW steel pipes makes it an ideal choice. Ordinary carbon electric resistance welded (ERW) pipes are mainly composed of carbon, with a small amount of manganese, silicon, sulfur and phosphorus added. These pipes have excellent formability and weldability, and are very suitable for the cold deformation shape of automobile body frames and rear body frames. The most common applications of ordinary carbon resistance welded pipes for automobiles are steering links, steering columns, shock absorbers, propeller shafts, tie rods, body frames, rear body frames and axle tubes.

The application of stainless steel in the automotive industry can be roughly divided into five categories: stainless steel for automotive exhaust systems, stainless steel for automotive fuel tanks, stainless steel for automotive frames, automotive stainless steel parts and stainless steel for automotive decoration.

Stainless steel for automotive fuel tanks is mainly stainless steel plate, which requires excellent stamping performance, welding performance and high corrosion resistance (internal fuel corrosion resistance and external corrosion resistance to harsh environments). Austenitic stainless steel, such as SUS304L, is used more. Stainless steel for automotive frames is high-strength stainless steel plate for automobiles, such as the integral body shell made of ferritic stainless steel, and its service life is generally 15-20 years.

Galvanized steel pipe is a steel pipe with a layer of zinc on the surface to improve its corrosion resistance. They are often used in parts such as chassis and body frames, which are exposed to the external environment and are easily eroded by factors such as rain and soil. The galvanized layer can effectively extend the service life of these parts. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion and long service life. The hot-dip galvanized steel pipe matrix undergoes complex physical and chemical reactions with the molten plating solution to form a corrosion-resistant and compact zinc-iron alloy layer. The alloy layer is integrated with the pure zinc layer and the steel pipe matrix, so it has strong corrosion resistance.

With the continuous development of the automotive industry, lightweighting has become an important trend. High-strength steel pipes have high yield strength and good ductility, which can reduce vehicle weight while maintaining strength. This type of steel pipe is widely used in important structural parts such as body frames and door anti-collision beams, effectively improving the collision safety performance of vehicles.

The steel portfolio of the automotive industry includes a series of high-strength grades of steel. The industry currently focuses on improving fuel efficiency, lightweight structures and reducing material costs, but this requires the use of ultra-high-strength formable steel. To achieve the required high strength, multiphase steels have been relied on since the 1990s.

To achieve the required stiffness, cold rolled PHS steels are heated at high temperatures and quenched in a steel die. This process is known as hot stamping or press hardening. Two different hot stamping methods are generally used: direct and indirect quenching. The blanks are first cut from medium-strength and high-ductility steel coils. In the indirect process, the blanks are preformed by cold pressing before heating in a heat treatment furnace; in direct press hardening, this step is omitted and the stamped steel sheets are heated directly. The steel is heated to a temperature of around 900-950°C. At this high temperature, the strength of the boron steel drops to around 200 MPa and excellent formability is achieved. The blank or preformed part is then transferred to a press forming process, where the steel is formed from the blank (direct hot stamping) or, in the case of preformed parts in the indirect method, the final forming is carried out in a cooling die and the quenching is completed until the temperature reaches about 200°C. The combination of hot stamping and rapid quenching achieves a tensile strength of 1500 MPa. The final step: cleaning and cutting the hardened steel sheet or part. The direct quenching process omits the preforming stage and is therefore more economical, more efficient and simpler and faster. It also exposes the steel to high deformation rates at high temperatures, although this also leads to correspondingly higher strains in the steel coating.

To protect the steel from oxidation and corrosion during the high-temperature quenching process, the surface of PHS steel is usually coated with a protective coating. The most common protective coatings are galvanized and aluminized coatings.

Protective coatings are applied to the surface of PHS steel to avoid surface damage and corrosion during the quenching process. Micro cracks will occur in all coatings, but not all coatings offer the same resistance to oxidation during subsequent processing and corrosion protection.

In automobile manufacturing, steel tube materials must not only meet the requirements of lightweight and high strength, but also cope with complex forming and anti-corrosion processes. Through the application of direct stamping hardening technology and protective coatings, various types of steel tubes have demonstrated excellent performance in the automotive industry. Whether it is seamless steel tubes for key transmission components or high-strength steel tubes for body frames, advances in steel tube technology have driven the automotive industry's continued innovation in performance, safety and environmental protection.