TRIP Steels (Transformation Improved Plasticity) – From “Bumper Component Welding State-of-the-Art Survey”
by D. W. Dickinson
AISI Bumper Project Group,Dec31, 2000
With some steels and under some continuous annealing conditions[1] as represented in this figure, a small amount of retained austenite (usually under 10%) can be present after completion of the anneal cycle. Note that with air cooling a larger amount of retained austenite is present in this steel (a little greater than 8 % compared to a little less than 2 % with the water quench). With subsequent cold working such as in cold rolling or forming operations during manufacturing, this retained austenite will transform to martensite reducing the amount of retained austenite as seen in this figure. This can lead to a dramatic increase in formability for manufactured parts as well as strength increase in this finished product.
A practice of interrupted quench from continuous anneal has also been experimentally tried with higher chemistry materials as illustrated in the above figure. The isothermal hold results in a structure variation from a) Martensite plus Retained Austenite, to b) Bainite Plus Retained Austenite, to c) Bainite with increased isothermal hold at elevated temperatures. These structures, particularly the retained austenite, also result in some very beneficial properties of these steels. Likewise, the temperature of the isothermal hold can have an effect on these structures as illustrated in the continuous cooling transformation curve in the figure above.
The volume fraction of ferrite, austenite and even bainite as well as the carbon content of each of these phases varies throughout the continuous anneal cycle as illustrated in this figure. As the material cools from the anneal, the volume fraction of austenite reduces while the carbon content in the retained austenite increases. At room temperature, relatively high carbon austenite (which can convert to high carbon martensite on deformation) results.
The resulting increase in ductility (uniform elongation) with the amount of retained
austenite[2] is presented above. Note the large increase in ductility when comparing dual phase steel characteristics (lower volume percent of retained austenite) and those for the TRIP steels (higher volume percent retained austenite). In general, increased austenite increases the ductility. To some extent, the strength is controlled by grain size as a direct result of the precipitate pinning mechanism discussed previously. The strength increase with niobium precipitation is also illustrated above
Thus, control of chemistry and cooling practices[3] can result in significant quantities of retained austenite. In particular, Mn promotes higher amounts of retained austenite by lowering martensite transformation temperature, and Si helps to increase stability of this retained austenite
. This figure illustrates the beneficial effect that these have on properties. Here the product of the tensile strength times the percent elongation is plotted as a function of the volume percent retained austenite with variations of Manganese and Silicon content showing the benefit on mechanical properties.
Although the TRIP Steels offer some interesting property characteristics of interest to high strength high formability manufacture such as the manufacture of bumpers and components, they have not found a significant amount of use in this industry to date. One of the reasons is the lack of weldability data on these steels. A quick examination of the chemistry of these TRIP steels reveals that they have higher chemistries than some of the other high strength steels. This equates to higher carbon equivalent values for these steels. In general, higher carbon equivalents cause a concern about potential reduced weldability. Many steel mills are limiting the carbon content of these steels to less than 0.2 % carbon because of this potential concern, however, this is an area where some actual weld testing is required before a complete weldability picture can be developed.
[1]ASM Handbook, Vol 1, 1990
[2]Hulka, K “Relationships between heat treatment conditions, microstructure and properties of Niobium microalloyed TRIP steel”, 41st MWSP Conf Proc. 1999
[3]Hulka, K “Relationships between heat treatment conditions, microstructure and properties of Niobium microalloyed TRIP steel”, 41st MWSP Conf Proc. 1999