Dual-phase steel
As a DP steel ( dual-phase steel ), all steels referred whose microstructure of a ferritic ( soft ) matrix, in which a predominantly martensitic ( strength-increasing ) the second phase is an island shape incorporated at the grain boundaries. The fraction of martensite is usually a maximum of 20 %. Having a higher martensite greater tensile strength can be achieved, but the elongation at break decreases. By this ambivalent structure combination, the dual-phase steel has a relatively low and thus favorable for the forming process yield with a high tensile strength. The low yield strength, which can be equated with an early plasticization of the material, is particularly important for complex components of great advantage. Typical applications for these steels are flat stretch-formed profiles in car doors, strength -related support structures and energy absorbing components, and steel wheels.
For the development of this new lightweight steels primarily a alloying of manganese ( 0.8-2 %), silicon ( 0.1-0.5 %) and aluminum (up 0.2%) and a novel heat treatment was responsible. Firstly, the above elements are lighter than iron, forcing the other hand it into certain structures which contribute to the crystal lattice with high ductility. The heat treatment in the two-phase region of ferrite and austenite ( see figure) causes the typical two-phase microstructure of ferrite and martensite. Martensite is produced by the rapid cooling from austenite, while ferrite remains approximately unchanged in its structure.
The idea for the production of dual phase steels dates back to the 1970s. During this time, by heating the ferritic- pearlitic steel in the two-phase region ( see figure, α γ region ) with subsequent rapid cooling ( coiling temperature RT - 350 ° C) discovered a new type of steel with until then stole unusual properties. By rapid cooling of the steels, the microstructure transformation of austenite into martensite is possible. The industrial production of DP steels began in the 1990s.
Today we know that dual phase steels (marked in the figure with a blue dot ) both by a conventional cooling from the pure γ - field as produced by the cooling from the α γ region ( marked in the figure with a red asterisk ) can be. The increased proportions of manganese, silicon and partly chrome cause the transformation of austenite to pearlite is delayed and thus a proportional transformation to martensite and tempered martensite is also achieved at lower cooling rates.
The dual-phase steels are standardized in the European standard EN 10346 (formerly EN 10336 ). They are not to be confused with duplex steels, consisting of equal parts of ferrite and austenite.