Interstitial-Free Steels, also known as IF steel, sometimes also called ultra-low carbon steel, have excellent deep drawing properties. In IF steel, due to the low C and N content, a certain amount of Ti and Nb are added to fix the C and N atoms in the steel into carbides and nitrides, so that there are no interstitial atoms in the steel, so it is called Interstitial-free steel. Usually the atoms of added elements in steel are dissolved into the lattice lattice of steel in two ways: interstitial and solid solution. When forming interstitial solid solution, interstitial atoms must be smaller than iron atoms, so they are easier to move between iron atoms. At the same time, iron atoms in steel have more lattice defects and dislocations, and interstitial atoms are easier to concentrate in these. Location. When steel deforms, iron atoms are displaced due to stress, and dislocations also move. If there are interstitial atoms at the dislocation, it will require a lot of energy to move. Compared with the dislocation movement without interstitial atoms, the deformation will be smaller, causing uneven deformation and reducing the plasticity of the steel.
Interstitial free steel properties
Interstitial-free steel has excellent deep drawing properties and is widely used in body panels with particularly complex shapes and large strains. For example, side outer panels, rear wheel housing inner panels, front fenders, etc.
As early as the 1960s, some people found that adding a certain amount of Ti to low carbon steel will combine with the C and N interstitial atoms in the steel to form precipitated particles, so that the low carbon steel can obtain solid solution strengthening. The low carbon steel will obtain excellent properties. Deep drawing performance. However, the carbon content of the steelmaking process at that time could only be controlled at 0.05wt%, and there was no means to control the N content. At this time, the C and N interstitial atoms in the solid solution steel needed to be added with a large amount of Ti. The cost was too high and there was no industrial production conditions. From the 1960s to the 1970s, smelting technology developed greatly, especially the promotion and application of vacuum degassing technology in metallurgical production, which allowed the C content in steel to be easily reduced to less than 0.01wt%. At the same time, N can also be effectively controlled. At this time, the cost of adding Ti to produce Ti-IF steel was greatly reduced, and the role of Nb in improving deep drawing performance was also discovered and applied. IF steel was officially launched as the third improvement in the stamping performance of automotive steel plates. IF steel is widely used in the automobile manufacturing industry, especially automobile outer panels and inner panels, which require good deep drawing performance to make it easy to form.
Deep drawing steel can be divided into commercial grade (CQ), ordinary stamping grade (DQ), deep drawing grade (DDQ), extra deep drawing grade (EDDQ) and super deep drawing grade (SUPER-EDDQ) according to the stamping grade. They are respectively Corresponds to several stages of the development of deep drawing steel. The development and application of the first generation of deep drawing steel products was ordinary boiling steel in the 1950s and 1960s, which could only be used for ordinary deep drawing parts; low carbon aluminum-killed steel was the second generation product, produced in the 1960s. , 1980s, with better deep drawing performance; after the 1980s, the third generation of ultra-low carbon ultra-deep drawing steel represented by IF steel appeared. In recent years, research on IF steel has found that slightly increasing the content of Mn, P, Si and other elements can improve the mechanical properties of IF steel while maintaining the good formability of IF steel. Ti, Nb and B also have the effect of improving the strength of IF steel, and the improvement of steel plate strength plays an important role in reducing the weight of automobiles and reducing material consumption. Therefore, the development and application of high-strength IF steel has become a new hot spot in the development of deep drawing steel.
High-strength IF steel is a solid-solution strengthened steel. The strength is mainly improved by adding solid-solution strengthening elements such as P, Mn, and Si to interstitial-free steel (IF steel). At the same time, since C and N atoms are completely fixed, there are no interstitial atoms. existence, the favorable texture develops preferentially during the annealing process, so it has good deep drawing performance. Because high-strength interstitial-free steel has both high strength and deep-drawing properties, it can be processed into parts with complex shapes, improve the dent resistance of automobiles, reduce the weight of automobiles, and meet the requirements of automobile safety, weight reduction, energy conservation and environmental protection.
The development of high-strength IF steel is in line with the requirements of vehicle weight reduction and high corrosion resistance in the automotive industry. The main strengthening mechanisms of high-strength steel plates are: ① solid solution strengthening; ② precipitation strengthening; ③ structural strengthening (phase transformation strengthening); ④ fine grain strengthening; ⑤ deformation strengthening. High-strength steel plates with different tensile strengths and elongations can be obtained using different metallurgical processes and strengthening mechanisms. High-strength IF steel achieves solid solution strengthening through alloying with elements such as Mn, P, and Si. The strength is improved without affecting the ductility and r-value. Nb-Ti alloyed IF steel adds phosphorus, silicon, manganese and boron to control cold work embrittlement. Compared with ordinary aluminum-killed steel or titanium alloyed IF steel, the disadvantage of this steel is delayed recrystallization, so to obtain the required texture and ductility, a sufficiently high recrystallization annealing temperature is required. High-strength IF steel has a high r value and n value. Among steels with medium tensile strength, high-strength IF steel is the best. High-strength IF steel plates are mainly used to make automobile interior panels after galvanizing.
The use of high-strength steel plates and thickness reduction are important means and directions for weight reduction, energy saving, safety, and environmental protection of automobiles. High-strength IF steel has broad development and application prospects.
