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Characterization of transformation induced plasticity steels for use in lightweight construction in the automotive industry
2003 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Transformation Induced Plasticity (TRIP) steels are multiphase steels including ferrite, bainite and retained austenite, which give them their excellent combination of high strength and high ductility. Therefore TRIP- steels represent formability and good crash worthiness and are mainly used in lightweight construction of car bodies in the automotive industry. The purpose of this work was to characterize TRIP-steels (TRIP700 and TRIP800) from different manufacturers (Arcelor, TKS and VoestAlpine) and with different coatings and to find a correlation between the microstructure and the mechanical properties as well as to study the crash behavior. The used methods were mainly laboratory tests, uniaxial tensile tests and bend tests, but also x-ray diffraction a magnetic method for the determination of the amount of retained austenite. To determine the crash behavior, studies with hat profiles were performed. There are mainly three alloying concept for TRIP-steels, all including carbon and manganese combined with only aluminum, only silicon or aluminum and silicon together for obtaining the multiphase microstructure with metastable retained austenite and the high strength and ductility. The amount of retained austenite is approximately 16-19 vol.% in delivery condition and decreases with increasing deformation level. The mechanical properties of TRIP-steels differ a lot. The yield and tensile strengths are for TRIP700 in the range from 440-585 MPa respectively 700-740 MPa and for TRIP800 500-650 MPa respectively 810-850 MPa. The materials with organic coatings have too high yield strength values (higher than norm values) caused by the bake hardening effect (increase of strength during heat treatment) that occurs during coating. The bake hardening effect has been determined for several of the studied TRIP-steels and the BH2 values are about 70-80MPa where the influence from the heat treatment is approximately 50% of the increase of strength. A study of reference body components (B-pillar) made of TRIP700Z TKS and TRIP800ZE Arcelor showed a good correlation between the amount of retained austenite and the mechanical properties. More as the retained austenite is transformed into martensite the higher is the yield and tensile strength. The crash behavior of some TRIP steels and a reference material (H320LA) has been determined by laboratory tests and by crash, crush and bend tests of hat profiles. The base material (and not the joining) has been given points for the crack behavior during crash, crush and bending. The material’s order became H320LA, TRIP700ZE+OC VoestAlpine, TRIP700ZE+OC Arcelor, TRIP800ZE+OC VoestAlpine and TRIP800ZE Arcelor, where the latter showed a rather bad crack behavior whereas the booth TRIP700 were quite good. Concerning the energy consumption during crash, the TRIP800 consumes more than the TRIP700 that consumes more than the reference material (H320LA). By using TRIP instead of H320LA the weight could be about 15-20% lower but of course the worse crack behavior has to be considered. To be able to use TRIP-steels in an adequate way for lightweight construction in the automotive industry more studies are required. One of the big problems today is that a TRIP-steel from one manufacturer can be totally different from a TRIP-steel from another one. Therefore just to say TRIP700 or TRIP800 are dangerous because a TRIP700 cannot always be changed to another TRIP700 and problems in the production may occur. TRIP-steels with their excellent properties are something for the future’s lighter vehicles and will probably widely be more and more used.

Place, publisher, year, edition, pages
Keyword [en]
Technology, high strength steel, TRIP-steel, Retained austenite, crash
Keyword [sv]
URN: urn:nbn:se:ltu:diva-58691ISRN: LTU-EX--03/324--SELocal ID: f43fb4a1-8c11-4a17-8193-c8533d58234fOAI: diva2:1032079
Subject / course
Student thesis, at least 30 credits
Educational program
Materials Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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