Thermomechanisch hergestellter, extrem fester Bainit mit reduziertem C-Gehalt
Final Report Abstract
In the frame work of this project, low temperature carbide-free bainitic steels with high strength and high toughness were produced in different groups of steels with medium to low carbon contents. The alloys were subjected to different processing conditions. The effects of the combination between the processing parameters and alloying elements on the structure development and mechanical properties were investigated. The development in this type of materials, compared with the conventionally produced/studied ones, is introduced by applying the following new modifications on the alloys and their processing conditions by: 1- Decreasing its carbon content: this has facilitated accelerating the bainite transformation and obtaining fine bainite structure in shorter time frames. Consequently, it was also possible to use continuous cooling in the bainite region using reasonable cooling rates to obtain the fine bainite structure. Decreasing the carbon content below the eutectoid composition enabled introducing the polygonal ferrite phase in the structure. The introduction of the ferrite phase is only significant for materials to be loaded under compression. The materials containing ferrite recorded inferior tensile properties compared to thus ferrite free ones. Decreasing the carbon content increases the produced bainite percentage on the expense of the retained austenite (γr) content. Additionally, an expected effect, which is behind the frame work of this project, is the improved formability and weldability. 2- Two methods were applied on the studied materials to obtain different generations of bainite. The first one was a two-step transformation process; the second one was a continuous cooling process in the bainite region. The highest yield strength was recorded for the two-step transformed material. The highest ductility was recorded for the continuously cooled material, applying the slowest used cooling rate (0.003 Ks-1). 3- Thermo-mechanical (TM) processing: The TM-simulators were used, enabling the study of the transformation kinetics Moreover the structure was designed by rolling samples down to 2 mm sheet material, using a rolling mill and reheating furnaces. Other side aspects were considered during this project, namely: 4- Studying the effect of manganese content on the structure development and the mechanical properties. It could be proved that redesigning the alloy by partial replacing the manganese with molybdenum improves the strength and accelerates the transformation without impairing the hardenability. 5- Studying the effect of the homogenisation process on the transformation kinetics and mechanical properties. It could be stated that the homogenisation process results in a slight increase in compression strength (CS) and a slight decrease in fracture strain (FS). The homogenised material recorded a range of CS from 2.1 to 2.3 GPa with FS ranging from 0.655 to 0.785, whereas the non-homogenised material performed a range of 2.0 to 2.2 GPa for CS with FS of 0.66 to 0.80. Designed alloy with 0.42 %C (corresponding to A2 in the proposal) is promising if compared with A1 and A3 (having 0.32%C and 0.56 %C, respectively) because of its wide spectrum of mechanical properties and its good combination of strength and ductility. The ductility of the material, in terms of total elongation (TEl) ranged from 0.04 to 0.2 in tension with an ultimate tensile strength (UTS) range of 1.53 to 2.08 GPa. A best combination of tensile properties is recorded for transformation at 290°C with UTS of 1.92 GPa and TEl of 0.2. The designed alloy with minimised Mn-content (material group C in the proposal) is promising if compared with the other ones having higher Mn-content, because of its higher strength and faster kinetics of bainite transformation without inferior ductility and hardenability. Applying deformation suppresses the bainite transformation for the alloys of group B. So it was not possible to obtain bainite in the TM processed material. Decreasing the cost of the produced steel by decreasing/eliminating the costly alloying elements, i.e. cobalt, chrome and nickel and redesigning the alloy and the process to substitute for their effects can be a strategy for widespread use of this type of material. Furthermore, studying the behaviour of this novel material under dynamic loading is of prime interest. The probable Improvement of the fatigue strength through decreasing the yield- to tensile-strength ratio of the ferrite containing material is to be investigated.
Publications
- Low temperature bainite in steel with 0.26wt% C. (2010) Materials Science and Engineering A, 527 (29-30), pp. 7706-7713
Soliman, M., Mostafa, H., El-Sabbagh, A.S., Palkowski, H.
- Microstructures and mechanical properties of thermomechanically processed low temperature bainite with 0.4 wt%-C in steel. 2nd international conference super high strength steels SHSS, Peschiera del Garda, Italy, 2010
Mostafa, H., Soliman, M., Palkowski, H.
- Role of dilatometer in designing new bainitic steels. (2010) Advanced Materials Research, 89-91, pp. 35-40
Soliman, M., Asadi, M., Palkowski, H.
