TRIP steel
TRIP steels ( Engl. TR ansformation I nduced P lasticity , dt . "Umwandlungsbewirkte plasticity ") are modern, particularly high-strength steel alloys .
history
As early as 1927, martensite was precipitated for the first time in a steel with a metastable austenitic structure at −40 ° C through transformation, which led to increases in strength and elongation. Since these nickel-based steels were not suitable for applications for various reasons, the idea of choosing a steel with a ferritic matrix was born in 1990.
use
In comparison to conventional steel types, TRIP steels have a higher strength and at the same time good ductility. As a result, they enable the production of lighter components with a given required strength and ductility. These properties make TRIP steels particularly interesting materials for the automotive industry , in which they are increasingly used today.
TRIP effect
The TRIP effect is the special martensite formation during forming. The stress-induced martensite formation must be distinguished from the deformation-induced martensite formation, since the TRIP effect only sets in with the latter. This causes a simultaneous increase in hardness and formability in the event of plastic stress in product manufacture or use. The extent of the effect is mainly influenced by the inexpensive alloy elements aluminum and silicon . In addition, much more expensive alloying elements such as nickel can be substituted.
The material's own yield point is higher than that of comparable steels, since the added silicon enables solid solution strengthening . As soon as the plastic range is reached during a deformation or deformation, the metastable, carbon-rich austenite begins to transform into martensite due to deformation. As a result, the TRIP steel is specifically strengthened by the plastic deformation.
Alloy components
TRIP steel consists mainly of several phases of iron-carbon alloys ; Essentially made of ferrite , carbide-free bainite and 5–10% metastable, carbon-rich residual austenite , which is deformed and transformed into martensite. Typical alloy additives for austenitic steels, the so-called austenite formers nickel, chromium (with low contents), cobalt , carbon , manganese and nitrogen , are also common. The specialty of TRIP steels are higher alloy additions of silicon and aluminum, the addition of which influences or makes the so-called TRIP effect controllable.
Manufacturing
The desired steel alloy is first brought to the recrystallization temperature and left there for a certain time in order to remove existing lattice defects through the formation of new nuclei and grain growth. There are ferrite and austenite in the structure . The annealed structure is then quenched to the T B temperature (B = bainite), i. That is, the cooling rate must be above the critical one in order to avoid phase changes caused by diffusion. The following components are formed in the structure:
- ferrite
- carbide-free bainite (this is why silicon alloys are used to prevent carbide formation)
- high carbon austenite.
To calm the structure, it is held at temperature T B for a while and then quenched to room temperature. The following structure is formed:
- ferrite
- carbide-free bainite
- metastable high carbon austenite.
Martensite does not form at room temperature, since the martensite start temperature is below this when the carbon content in the austenite is high.
See also
- Dual phase steel
- TWIP steel
- Stacking fault energy (here: <20mJ / m²)
Web links
Individual evidence
- ↑ Julia Carolin Imlau: Relationship between microstructure, damage process and mechanical properties in TRIP steels , 2009, p. 1.