Alloy and structure design of austenitic cast Cr-Ni-Cu-N steel exhibiting TRIP/TWIP properties for cold massive forming

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The aim of the project is the conception, processing and research of a copper-alloyed, austenitic, stainless Cr-Ni-Cu-N cast steel with TRIP/TWIP properties used for process step reduced cold massive forming (without hot forming) by cross rolling into test samples. The addition of copper increases the stacking fault energy and improves the cold formability of austenite due to a decrease of the strain-induced martensite formation capability. Moreover, using the austenite stabilizing elements copper and nitrogen the alloying costs can be reduced by partial saving of expensive nickel. A further aim is to produce copper-containing precipitates which contribute to the precipitation hardening of the austenite and to define heat treatment parameters at which the formation of the ε-copper phase can be partially or completely suppressed. With the aid of tailored alloy design, the deformation mechanisms (TRIP/TWIP effect, planar glide) running in the austenite during cold forming can be triggered purposefully to guarantee an optimal cold formability. Furthermore, after cold forming operation an accompanying characterization of martensite formation capability and microstructure is planned. A further focus is placed on the processing and research of cold formed and recrystallized test samples made of Cr-Ni-Cu-N cast steel. By means of reversion annealing in the temperature range of 700 °C the previously coarse grained austenitic cast microstructure should be transferred in a(n) (ultra) fine-grained (FG, UFG) microstructure without hot forming. A further research focus is the extensive characterization of the temperature and strain rate dependent deformation behaviour as well as the cyclic deformation behaviour.Cold forming of cast steel material possesses the substantial advantage of a shorter process chain, which leads to a considerable energy saving, and the increase of productivity and economic efficiency.