TRIP steel

TRIP steel is a high-strength steel typically used in the automotive industry.^[1] TRIP stands for "Transformation induced plasticity." It is known for its outstanding combination of Strength and Ductility.

Microstructure

TRIP steel has a microstructure consisting of retained Austenite in a ferrite matrix. Apart from Retained Austenite it also contains hard phases like Bainite and Martensite.^[2] The higher 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.

Metallurgical Properties

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.^[3] High strain hardening capacity and high mechanical strength lend these steels excellent energy absorption capacity. TRIP steels also exhibit a strong bake hardening (BH) effect following deformation.^[4] 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 (DP) steel.

Effect of Alloying Elements

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.

Applications

As a result of their high energy absorption capacity and fatigue strength, TRIP steels are particularly well suited for automotive structural and safety parts such as cross members, longitudinal beams, B-pillar reinforcements, sills and bumper reinforcements

The most common TRIP range of steels comprises 2 cold rolled grades in both uncoated and coated formats (TRIP 690 and TRIP 780) and one hot rolled grade (TRIP 780), identified by their minimum tensile strength expressed in MPa.

References