Spoluautoři JIRKOVÁ Hana, KUČEROVÁ Ludmila, MAŠEK Bohuslav

Several innovative methods of heat and thermo-mechanical treatment have been recently developed to improve mechanical properties of high strength low alloyed steels. On the base of the experience obtained during the development of TRIP (transformation induced plasticity) steels, other methods have been proposed which also use positive effect of multiphase microstructure with controlled amount of retained austenite to achieve excellent strength to ductility ratios. Long term bainite annealing belongs among these methods which stabilize retained austenite in the final microstructure. Low alloyed CMnSi steel with 0.2 % C, 1.8 % Si a 1.5 % Mn was used for experimental program. Four different processing routes were used in experimental heat treatment of this steel. All of them had the same heating temperature of 1000 °C with 25 min hold at this temperature. Three different cooling rates were applied in subsequent cooling to either 150 °C or 300 °C. The cooling rates corresponded to cooling in water, air and in the furnace. When the desired temperature was reached, long term bainite annealing at this temperature was carried out, lasting either 12 or 24 hours. The choice of these very long annealing times at relatively low temperatures should enable the formation of ultra-fine bainite. Slow grow of bainite units should also influence homogeneity of carbon distribution within bainite laths. The resulting microstructures were analysed by laser scanning confocal microscopy and scanning electron microscopy. Mechanical properties were established by tensile tests. The above mentioned heat treatments resulted in the final microstructures consisting of various mixtures of free ferrite, pearlite, bainite and small amounts of martensite and retained austenite. The strength of these microstructures was in the region of 630 – 1216 MPa and ductility reached 19-44 %. The highest strength of 1216 MPa with ductility of 14 % was obtained after water cooling and 24 hours annealing at 150 °C. On the other hand, the lowest strength of 630 MPa with the highest ductility of 44 % were reached after furnace cooling and subsequent 12 hour annealing at 300 °C.