TRIP steels are high strength steels. TRIP stands for ‘transformation induced plasticity’. They are new generation of low alloy steels. These steels offer outstanding combination of strength and ductility as a result of their micro structure. TRIP steels rely on the transformation of austenite grains into the harder phase of martensite during deformation for achieving their mechanical properties. The locations of these grains in the microstructure are of major importance because they influence the impact of the TRIP effect, the microstructural localization and therefore the macroscopical deformability of the material.
Microstructure and composition
The microstructure of these steels is composed of islands of hard residual austenite and carbide free bainite dispersed in a soft ferritic matrix. The retained austenite is embedded in a primary matrix of ferrite. In addition to a minimum of 5 % to 15 % of retained austenite, hard phases such as martensite and bainite are present in varying amounts. Austenite is transformed into martensite during plastic deformation (TRIP effect), making it possible to achieve greater elongations and lending these steels their excellent combination of strength and ductility. Fig 1 shows the typical microstructure of TRIP steel.
Fig 1 Typical micro structure of TRIP steel
TRIP steels typically require the use of an isothermal hold at an intermediate temperature, which produces some bainite. The higher silicon and carbon content of TRIP steels also result 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 aluminum accelerate the ferrite/bainite formation. They are also added to avoid formation of carbide in the bainite region. Silicon though a key element for the formation of retained austenite, is undesirable to make steel sheet with high surface quality.
Altering the carbon content helps controlling the strain level wherein austenite starts transforming to martensite. At lower carbon levels, the transformation of the retained austenite begins almost instantaneously upon deformation, thus improving the formability and work hardening rate during the stamping process. On the other hand, the retained austenite has stability at higher carbon contents and its transformation starts only at strain levels above those applied during forming. The retained austenite remains in the final component at these carbon levels. Its transformation into martensite takes place during subsequent deformation, such as a crash event.
Properties of TRIP steels
TRIP steels are hot rolled, cold rolled, or hot dip galvanized products with strength ranging from 500 MPa to around 800 MPa. TRIP steel has high elongation and excellent sustainable work hardening ratio. Hence they are suitable for stretch forming.
These steels have high strain hardening capacity. They exhibit good strain redistribution and thus good drawability. As a result of strain hardening, the mechanical properties, and especially the yield strength, of the finished part are far superior to those of the initial blank.
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, which further improves their crash performance. The characteristics of TRIP steel are described below.
- Work hardening – As compared with other high strength steels, TRIP steel displays higher work hardening rate in entire range of plastic deformation.
- Yield point elongation (YPE) – Tested as delivered, TRIP steels usually show YPE. However, some grades may have no YPE.
- Formability – Due to high work hardening rate, TRIP steel behaves in a stable way in stamping processes (resistance to onset necking) and displays remarkably high formability (high potential to form parts of complex geometry).
- Bendability – TRIP steel demonstrates good bendability; As a result, product and process design solutions leading to spring back control are easier to implement.
- Bake hardening – TRIP steels have an excellent bake hardening capacity. The increase in the yield strength in a typical paint baking cycle is approximately 70 MPa.
- Product mass reduction capacity – TRIP steels have high potential for part down gauging and weight reduction.
- Fatigue performance – TRIP steels have higher fatigue strength than equivalent conventional HSLA (high strength low alloy) steels.
TRIP steels can be designed or customized to yield superior formability for fabricating intricate AHSS components or to demonstrate high work hardening in the event of crash deformation for outstanding crash energy absorption. The additional alloying requirements of TRIP steel deteriorate its resistance spot-welding behavior. This can be handled by altering the welding cycles employed, for instance, dilution welding or pulsating welding.
TRIP steels are produced by a forming operation, during which retained austenite transforms into martensite due to work hardening. The production process of TRIP steels has the following steps.
- The steel is heated to the dual phase region at temperature of 760 deg C to 780 deg C. During this treatment cold rolled steel sheet recrystallizes and carbon in cementite dissolves into austenite.
- Cooling into the bainite temperature range is performed, so that pearlite formation is prevented. Carbon partitions between the ferrite and austenite phases and for slow cooling rates the carbon content in austenite increases.
- In the isothermal region at 400 deg C to 450 deg C the transformation to bainite takes place. Formation of cementite is prevented due to the high amounts of silicon, aluminum and phosphorus used as alloying elements. Austenite is transformed into ferritic bainite and into carbon rich retained austenite, which remains in the structure.
After the thermal treatment of TRIP steels, a triple-phase microstructure is obtained, consisting of ferrite, bainite and retained austenite. Retained austenite is metastable at room temperature and transforms to martensite during straining. TRIP steels are essentially composite materials with evolving volume fractions of the individual phases. The total strain is assumed to be the sum of elastic, plastic and transformation parts.
These excellent mechanical properties arise from a martensitic transformation of metastable retained austenite, induced by external stress and/or plastic deformation. The TRIP steels possess a multi-phase microstructure, consisting typically of ferrite (?-Fe), bainite and retained (metastable) austenite (?-Fe). The microstructure is formed after inter critical annealing and a subsequent isothermal annealing in the bainitic transformation region, called austempering. The various levels of these phases give TRIP steels their unique balance of properties.
The carbon content in austenite is increased both during the inter-critical annealing and during the austempering. The carbon enrichment during austempering is the result of the suppression of the formation of carbides during the bainitic transformation, due to the presence of the alloying elements such as aluminum and silicon. The enrichment of carbon in the austenite increases its thermal stability and consequently, the austenite can be retained upon cooling to room temperature.
Comparison with DP steels
Like Dual Phase DP steels, the microstructure of TRIP steels is comprised of mainly soft ferrite. While DP steels have a microstructure with islands of hard martensite dispersed throughout, TRIP steels have a combination ferrite (?-Fe), bainite and retained (metastable) austenite (?-Fe).
TRIP steels have a lower initial work hardening rate when compared to DP steels; however the hardening rate sustains at higher strains where the work hardening rate of DP steels starts decreasing. The work hardening rates of TRIP steels are considerably greater than for traditional HSS, yielding significant stretch forming.
This quality is especially helpful to designers who leverage the high work hardening rate and increased bake hardening effect for designing a component using the as-formed mechanical properties. Since in TRIP steels, the high work hardening rate sustains at higher strains, these materials have a slight advantage over DP steels in the most rigorous stretch forming applications.
Applications of TRIP steels
Characteristics of TRIP steel make it especially useful for difficult to form parts. These components can benefit from the increased strength and strain rate sensitivity that TRIP offers. TRIP steels are suitable for structural and reinforcement parts of complex shape.
Because of the increased formability, TRIP steels can be used to produce more complicated parts than other high strength steels, thus allowing the automotive engineer more freedom in parts design to optimize weight and structural performance. In addition, these steels can be used for the automotive body structure to provide excellent crash energy absorption.
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.