TRIP steel

TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for 'Transformation induced plasticity,' which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility. TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. TRIP stands for 'Transformation induced plasticity,' which implies a phase transformation in the material, typically when a stress is applied. These alloys are known to possess an outstanding combination of strength and ductility. TRIP steels possess a microstructure consisting of austenite with sufficient thermodynamic instability such that transformation to martensite is achieved during loading or deformation. Many automotive TRIP steels possess retained austenite within a ferrite matrix, which may also contain hard phases like bainite and martensite.. In the case of these alloys, the high silicon and carbon content of TRIP steels results in significant volume fractions of retained austenite in the final microstructure. TRIP steels use higher quantities of carbon than dual-phase steels to obtain sufficient carbon content for stabilizing the retained austenite phase to below ambient temperature. Higher contents of silicon and/or aluminium accelerate the ferrite/bainite formation. They are also added to avoid formation of carbide in the bainite region. For use in naval and marine applications, both martensitic/austenitic and fully austenitic steels have been of interest due to their exhibited large uniform elongation, high strength, and high fracture toughness. These properties are exhibited because of a deformation-induced martensitic transformation from parent phase (FCC γ austenite) to the product phase (BCC α' martensite). This transformation is dependent on temperature, applied stress, composition, strain rate, and deformation history, among others. During plastic deformation and straining, the retained austenite phase is transformed into martensite. Thus increasing the strength by the phenomenon of strain hardening. This transformation allows for enhanced strength and ductility. High strain hardening capacity and high mechanical strength lend these steels excellent energy absorption capacity. TRIP steels also exhibit a strong bake hardening effect. Bake hardening is an increase in strength observed when work hardening during part formation is followed by a thermal cycle such as paint-baking. Research to date has not shown much experimental evidence of the TRIP-effect enhancing ductility, since most of the austenite disappears in the first 5% of plastic strain, a regime where the steel has adequate ductility already. Many experiments show that TRIP steels are in fact simply a more complex dual-phase steel. The amount of carbon determines the strain level at which the retained austenite begins to transform to martensite. At lower carbon levels, the retained austenite begins to transform almost immediately upon deformation, increasing the work hardening rate and formability during the stamping process. At higher carbon contents, the retained austenite is more stable and begins to transform only at strain levels beyond those produced during forming. The temperature at which a TRIP steel is stressed or deformed can be related to the martensitic start temperature (Ms). Applied stresses can assist in the transformation process by effectively adding an increased energy for transformation that allows for the martensitic transformation to occur above the Ms temperature. Above the Ms temperature, transformation behavior is temperature dependent and shifts from stress-induced to strain-induced at a temperature known as the Msσ temperature. The Msσ temperature is defined as the maximum temperature at which an elastic stress causes a martensitic transformation, initially defined by Richman and Bolling. Below Msσ, martensitic transformation is classified as stress assisted because transformation nucleates on pre-existing sites (e.g., dislocations, grain boundaries, phase boundaries, etc.), and the applied stress thermodynamically assists the transformation. At temperatures above Msσ, yielding and plastic deformation occur before transformation, and nucleation of martensite occurs at the intersection of shear bands created from the strain of the plastic deformation.