Papers by Keyword: Niobium Microalloying

Abstract: One of the main targets in automotive industry is to reduce the weight of vehicle as well as increase the safety. To accomplish this goal, press-hardening steel and hot stamping parts have been used in car body. However, the possibility of hydrogen-induced delayed fracture (HDF) of hot stamping parts exits, which will decrease the car’s passive safety. A solution has been presented to reduce the sensitivity of HDF and improve hydrogen-induced delayed fracture resistance (HDFR) by Niobium micro-alloying technology. Traditional press-hardening steel 22MnB5 and new steel 22MnBNb2, 22MnBNb5 and 22MnBNb7 were studied, and it is shown that the appropriate addition of Nb is beneficial to the improvement of the delayed fracture resistance of the hot stamping steel, which indicates that Niobium micro-alloying technology is an effective solution to the HDF in hot stamping steels.

Abstract: Modern vehicle bodies make intensive use of high strength steel grades to improve the weight and the mechanical performance simultaneously. A broad range of medium and extra high strength steel grades is available. These steel grades have different characteristics concerning strength, formability and weldability. For many steel grades microalloying by niobium is the key to achieve their characteristic property profile. In HSLA steels niobium enhances the strength primarily by grain refinement. In interstitial free high strength steels niobium serves as a stabilizing element and also assists in obtaining the bake hardening effect. Some modern multiphase steels rely on niobium to achieve additional strength via grain refinement and precipitation hardening. Microstructural control provides a way to further optimize properties relevant to automotive processing such as cutting, forming and welding. The relevance of niobium microalloying in that respect will be outlined.

Abstract: The use of thin slab casting and direct rolling is well suited for the production of niobium microalloyed low-carbon high strength linepipe grades. The slabs have excellent surface quality. Thermomechanical processing by controlling hot work hardening and softening processes of austenite and its polymorphic transformation into ferrite results in a powerful microstructure refinement. This is a sound basis for setting high strength, combined with excellent ductility and toughness.

Abstract: In the recent years several new high strength steel grades have been developed, which exhibit improved cold formability and thus are especially suitable for the car body. These multi-phase steels have to be processed via continuous annealing lines. The best combination of strength and uniform elongation is obtained with ferrite-bainite-retained austenite multiphase steel, which rely on transformation induced plasticity (TRIP effect) during the cold forming operation. The production route for TRIP steel, which relies on two isothermal heat treatments, is explained as well as the role of the major alloying elements. In order to optimize the property combinations, microalloying with niobium is a successful means. Niobium increases the strength of the steel with 15 MPa per 0.01 %Nb and simultaneously provides also a higher amount of stable austenite, thus guaranteeing both, increased strength and formability. The metallurgical background for providing these results is explained. Even though the amount of high strength steel in automobiles is continuously increasing, the application of TRIP steel is still limited. However, the good experience in trials and first successful applications make an increased usage of this steel type also realistic, despite its relatively high production costs and reduced weldability.