KUČEROVÁ Ludmila Západočeská univerzita v Plzni

Spoluautoři JIRKOVÁ Hana, PRŮCHA Vojtěch, PILEČEK Vít, MAŠEK Bohuslav

New strategy of thermo-mechanical treatment with continuous cooling was tested for two low alloyed TRIP (transformation induced plasticity) steels. TRIP steels are high-strength steels with good formability and ductility and this excellent combination of mechanical properties results from fine, multiphase microstructure with about 10-15% of stabilized retained austenite. The processing of TRIP steels typically consist of heating either into intercritical region or above, deformation applied during the cooling form this temperature and bainitic hold at the temperatures around 400°C. The hold should ensure convenient bainite content in the final microstructure and also sufficient carbon enrichment of austenitic areas. This aim of this work is to design and test new processing strategy, in which time consuming bainitic hold should be replaced by continuous cooling. Two steels with chemical composition typical for TRIP steels have been chosen for this experimental program, 0.2C-1.5-Mn-1.8Si and 0.2C-1.5Mn-1.8Si-0.059Nb. Reference samples were prepared from both steels, processed by thermo-mechanical treatment with incremental deformation steps and bainitic hold. The new strategy was based on the standard rolling mill processing of similar steels. Processing parameters were optimised with respect to the final microstructure and properties. Obtained microstructures were analysed with the help of light, scanning confocal microscopy and scanning electron microscopy. The amount of retained austenite was established by X-ray diffraction phase analysis, reaching the values around 15%. Mechanical properties were determined by tensile test, hardness measurement and notch impact test. New strategy of thermo-mechanical treatment resulted in ultimate strengths above 1000MPa with ductility above 20%.