TRIP steels have a microstructure consisting of at least five volume percent of retained austenite, which is embedded in a primary ferrite matrix. The microstructure also contains hard phases like bainite and martensite in varying amounts.
The use of an isothermal hold at an intermediate temperature is typically required for TRIP steels in order to create some bainite structure. The presence of retained austenite in large volume fractions in the final microstructure is a result of the higher levels of carbon and silicon in TRIP steels. A schematic of TRIP steel microstructure is depicted in Figure 1, and a micrograph of TRIP 690 is shown in Figure 2.
Figure 1. Bainite and retained austenite are additional phases in TRIP steels
Figure 2. Micrograph of TRIP 690 steel
Work Hardening Rates
Hard second phases are dispersed in soft ferrite during deformation, creating a high work hardening rate in DP steels. However, in the case of TRIP steels, increasing strain gradually transforms the retained austenite to martensite, causing an increment in the work hardening rate at higher strain levels (Figure 3). Although the initial work hardening rate of the TRIP steel is lower than the DP steel, the hardening rate of the TRIP steel sustains at higher strain levels at which the DP steel experiences a deterioration in the work hardening rate.
Figure 3. TRIP 350/600 with a greater total elongation than DP 350/600 and HSLA 350/450
TRIP steels have significantly higher work hardening rates than that of conventional HSS, enabling substantial stretch forming. This is especially helpful in designing components using the as-formed mechanical properties. The ability of maintaining the high work hardening rate at higher strain levels makes the TRIP steels a better choice over the DP steels in applications involving the most severe stretch forming.
Influence of Carbon Content in TRIP Steels
The carbon content in TRIP steels is much higher than DP steels in order to stabilize the retained austenite phase at temperatures below the ambient temperature. Higher contents of silicon and/or aluminum promote the formation of ferrite/bainite. Hence, these elements help in sustaining the required amount of carbon within the retained austenite. Hindering carbide precipitation during bainitic transition seems to be critical for TRIP steels. Carbide precipitation in the bainite region can be avoided through the use of elements such as aluminum and silicon.
Adjusting the carbon content helps controlling the strain level wherein austenite starts transforming to martensite. During deformation, the transformation of the retained austenite begins almost instantaneously at lower carbon levels. As a result, the formability and work hardening rate are improved during the stamping process. Conversely, the retained austenite has high stability at higher carbon levels 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 and its transformation to martensite takes place during crash deformation.
Engineering or customizing of TRIP steels is possible to achieve superior formability in the fabrication of intricate AHSS components, or provide high work hardening rate for better crash energy absorption in the event of crash deformation. The resistance spot-welding behavior of TRIP steels is deteriorated as a result of the additional alloying requirements. However, this issue can be handled by modifying the welding cycles employed, for instance, dilution welding or pulsating welding. Figure 4 shows the tensile strength-elongation graph of the TRIP steels.
Figure 4. The tensile strength-elongation graph of TRIP steels.
The following table summarizes the current production grades of TRIP steels and corresponding automotive applications:
||Frame rails, rail reinforcements
||Side rail, crash box
||Dash panel, roof rails
||B-pillar upper, roof rail, engine cradle, front and rear rails, seat frame
Download the Advanced High-Strength Steels Applications Guidelines free here, to learn more about the metallurgy, forming and joining of these new steels.
This information has been sourced, reviewed and adapted from materials provided by WorldAutoSteel (World Auto Steel).
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