Thermomechanisch hergestellter, extrem fester Bainit mit reduziertem C-Gehalt

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A novel method for producing extremely strong and cheap steel is under development. The steel is reaching strength up to 2500 MPa, a hardness of 600-670HV and fracture toughness in excess of 30-40MPam1/2. The new material relies on microstructure called bainite which is known since more than seven decades; the novelty is in the alloy design which leads to a fine structure and controlled response to heat treatment. The high strength of these steels is due to their very fine structure. The observed refinement is a consequence mainly of the ability of high carbon content and low transformation temperature to enhancing the strength of the austenite. It is expected that the bainite-plates will become thinner as the yield strength of the austenite, from which they are formed, increases. Thus, the high carbon directly refines the structure by strengthening the austenite and indirectly by lowering the Ms temperature and thus allowing performing the bainitic transformation process at lower temperature. However, the high carbon content results in the existence of large regions of untransformed austenite in the microstructure, which is known to be detrimental to the mechanical properties. The T0 curve, which limits the amount of bainite that can be formed at any temperature, is the reason for the existence of these large regions of untransformed austenite. Our first pre-tests results showed that decreasing the carbon content has the following advantages: - Accelerating the bainite transformation reaction. - Releasing the cementite in a certain temperature range in the intercritical region which provided the possibility of intercritical annealing of the alloys. This possibility results in a wider range of mechanical properties. - The observed tensile and compression properties are comparable with those reported in ref. [1, 6]. This is justified by the decrease in the retained austenite content as a result of decreasing the carbon content. Through out the work of this project the following points will be investigated: - Investigation of the effect of alloying elements on the phase transformation process and mechanical properties. - Lowering the carbon content and simultaneously substituting the decrease in austenite strength by thermomechanical processing (10). This development will result in obtaining this type of microstructure in flat products.