Ultrafine grained high-alloyed austenitic TRIP steel

High-alloyed cast TRIP steel showing pronounced martensitic phase transformation during plastic deformation like tensile, compressive or cyclic loading exhibits concurrently high strength and high ductility. Further increase in the yield strength/ultimate tensile strength can be realized by a smaller austenitic grain structure. An ultrafine grain size of the austenite can be realized among others by well situated thermo-mechanically controlled processing like combination of cold rolling and subsequent specific heat treatment. Cast plates of high-alloyed TRIP steel were cold rolled to different deformation degrees. Subsequently, heat treatment experiments at different annealing temperatures and annealing times were performed. The microstructures after heat treatment were investigated by X-ray diffraction and scanning electron microscopy. The obtained grain size of the reverted austenite was determined by EBSD measurements. Finally, tensile tests on reverted austenitic steel specimens were performed in order to determine the influence of the grain size on the mechanical properties. The results show that plates of a high-alloyed cast TRIP steel can be cold rolled up to 90% of thickness reduction leading to high amount of α′-martensite (80%). Specific heat treatment of 90% cold rolled TRIP steel results in ultrafine grained reverted austenitic microstructure with mean grain size of about 1 μm. Tensile tests of heat treated steel specimens revealed an enormous increase in the yield strength up to 1000 MPa. The ultrafine grained austenitic steels still show the TRIP effect at a grain size of 1 μm.