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.

Microstructure

TRIP steels possess a

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

consisting of

austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 ...

with sufficient thermodynamic instability such that transformation to

martensite Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties Ma ...

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 Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face ...

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

phase to below ambient temperature. Higher contents of

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

and/or

aluminium Aluminium (aluminum in AmE, American and CanE, Canadian English) is a chemical element with the Symbol (chemistry), symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately o ...

accelerate the ferrite/

formation. They are also added to avoid formation of

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...

in the

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.

Metallurgical properties

During plastic deformation and straining, the retained

phase is transformed into

. Thus increasing the strength by the phenomenon of

strain hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strength ...

. This transformation allows for enhanced strength and

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

. 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.

Effect of alloying elements

The amount of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

determines the strain level at which the retained

begins to transform to

. At lower carbon levels, the retained

begins to transform almost immediately upon deformation, increasing the

work hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strength ...

rate and

formability Formability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experienc ...

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.

Effect of temperature

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.

Effect on mechanical properties

The TRIP effect can be exploited to extend the uniform plastic ductility by delaying the onset of necking, thereby delaying the flow localization instability that follows the formation of a stable neck. The formation of a stable neck can be defined as when the fractional increase in true stress is equal to the fractional decrease in load-bearing area of a sample. This can also be described as the point at which the strain hardening rate in an engineering stress-strain curve becomes negative. This can be explained by a power law equation for stress-strain behavior for plastic flow: Where n is the strain hardening coefficient, is the stress, is the strain, and K is the strength coefficient. By this equation, stable plastic flow is maintained by maintaining a minimum strain hardening coefficient, which can be expressed by: This exponential strain hardening behavior represents the optimal curvature of the stress-strain curve while maintaining a minimum n for stable nonlocal plastic flow. It has been observed that TRIP steels exhibit this exponential strain hardening behavior when deformed at a temperature near to and above , thereby displaying an optimum in uniform plastic ductility.OLSON, G. B., "Transformation Plasticity and the Stability of Plastic Flow," pp. 391–424, ASM, 1984. By this observation, the TRIP effect can reverse the curvature of stress-strain behavior, and this reversal drives significant improvement in uniform ductility.

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 TRIP effect can also be utilized in forming operations, where improvements to ductility enable greater bend angles and more aggressive forming operations without cracking. 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 ultimate tensile strength expressed in MPa. TRIP steels are well suited to armor applications, where increases in uniform ductility (and therefore ballistic energy absorption) can improve protection against projectiles and ballistic threats while maintaining or reducing plate thicknesses.

References

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