TRIP steel

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TRIP steel are a class of high-strength steel alloys typically used in naval and marine applications and in the automotive industry. [1] 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.

Contents

Microstructure

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. [2] 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. [3]

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. [4] High strain hardening capacity and high mechanical strength lend these steels excellent energy absorption capacity. TRIP steels also exhibit a strong bake hardening effect. [5] 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 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.

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. [6] 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. [7] 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. [8]

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:

σT=K(εT)n

Where n is the strain hardening coefficient, σT is the stress, εT is the strain, and K is the strength coefficient. [9] By this equation, stable plastic flow is maintained by maintaining a minimum strain hardening coefficient, [10] which can be expressed by:

n=σ0exp(ε)

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 Msσ, thereby displaying an optimum in uniform plastic ductility. [11] 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.

Related Research Articles

Steel Metal alloy of iron with other elements

Steel is an alloy made up of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant typically need an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, weapons, and rockets. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Heat treating Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Martensite Type of steel crystalline structure

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.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

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 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

Bainite

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

Martensitic stainless steel One of the 5 crystalline structures of stainless steel

Stainless steels can be classified by their crystalline structure into five main types: austenitic, ferritic, martensitic, duplex, and precipitation hardened. Martensitic stainless steel is a specific type of stainless steel alloy that can be hardened and tempered through multiple ways of aging/heat treatment.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

In metallurgy, a shape-memory alloy (SMA) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It may also be called memory metal, memory alloy, smart metal, smart alloy, or muscle wire.

Maraging steel

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

Tempering (metallurgy) Process of heat treating used to increase toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.

Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

Dual-phase steel Type of steel with a ferritic–martensitic microstructure

Dual-phase steel (DP steel) is a high-strength steel that has a ferritic–martensitic microstructure. DP steels are produced from low or medium carbon steels that are quenched from a temperature above A1 but below A3 determined from continuous cooling transformation diagram. This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content. For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement). The desire to produce high strength steels with formability greater than microalloyed steel led the development of DP steels in the 1970s.

Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point and then quenched in a salt, oil, or lead bath kept at a temperature of 150-300 °C. The workpiece is held at this temperature above martensite start (Ms) point until the temperature becomes uniform throughout the cross-section of workpiece. After that it is cooled in air or oil to room temperature. The steel is then tempered. In this process, Austenite is transformed to martensite by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

The R-phase is a phase found in nitinol, a shape-memory alloy. It is a martensitic phase in nature, but is not the martensite that is responsible for the shape memory and superelastic effect.

Thermomechanical processing is a metallurgical process that combines mechanical or plastic deformation process like compression or forging, rolling, etc. with thermal processes like heat-treatment, water quenching, heating and cooling at various rates into a single process.

Twinning-Induced Plasticity steel which is also known as TWIP steel is a class of austenitic steels which can deform by both glide of individual dislocations and mechanical twinning on the {1 1 1}γ<1 1 >γ system. They have outstanding mechanical properties at room temperature combining high strength and ductility based on a high work-hardening capacity. TWIP steels have mostly high content in Mn and small additions of elements such C, Si, or Al. The steels have low stacking fault energy at room temperature. Although the details of the mechanisms controlling strain-hardening in TWIP steels are still unclear, the high strain-hardening is commonly attributed to the reduction of the dislocation mean free path with the increasing fraction of deformation twins as these are considered to be strong obstacles to dislocation glide. Therefore, a quantitative study of deformation twinning in TWIP steels is critical to understand their strain-hardening mechanisms and mechanical properties. Deformation twinning can be considered as a nucleation and growth process. Twin growth is assumed to proceed by co-operative movement of Shockley partials on subsequent {111} planes.

References

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