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TRIP Steels
TRIP steels are one of the newest and most exciting materials being developed by the steel industry. In relation to other advanced high-strength steels, TRIP steels exhibit better ductility at a given strength level. This enhanced formability comes from the transformation of retained austenite (ductile, high temperature phase of iron) to martensite (tough, non-equilibrium phase) during plastic deformation. In fact, the acronym “TRIP” stands for TRansformation Induced Plasticity. Because of this increased formability, TRIP steels can be used to produce more complicated parts than other high strength steels allowing the automotive engineer more freedom in part design to optimize weight and structural performance.
Comparison of DUAL-TEN®
(dual
phase) steel vs. TRIP steel
| DUAL-TEN®
Steel |
Topic
Area |
TRIP
Steel |
| Ferrite-martensite |
Microstructure |
Ferrite-bainite-austenite |
| None |
Yield
Point Elongation |
Yes |
| Good |
Formability
for strength level |
Very
Good |
| Excellent |
Bake
Hardening |
Excellent |
| Good |
Strain
Rate Sensitivity |
Excellent |
| Good |
Fatigue |
Good |
| OK |
Weldability |
More
difficult than DUAL-TEN® |
The 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.
Typical Grades:
Various tensile strengths from 500 MPa to 800 MPa
Typical Microstructure:
The microstructure of unformed TRIP steel consists of varying amounts of ferrite, bainite, and retained austenite depending on the strength level desired.
Characteristics of TRIP steels:
- 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 bendabilty. As a result, product and process design solutions leading to springback control are easier to implement.
- Bake hardening – TRIP steels have an excellent bake-hardening capacity. The increase in the yield strength in typical paint baking cycle is approximately 10 ksi (70
MPa).
- Product mass reduction capacity – TRIP steels have high potential for part downgauging and weight reduction.
- Fatigue performance – TRIP steels have higher fatigue strength than equivalent conventional HSLA steels.
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