| Grain refinement strengthening, dislocation strengthening, second phase strengtheningand solid solution strengthening are the main strengthening mechanisms in martensiticsteels. To carbon steels, microstructure parameters, such as dislocation densities, grain size,and retained austenite content, all closely relates with the carbon content. Thus carboncontent is the most important factor controlling the martensite strength. Carbon segregatesat the defects of dislocations and grain boundaries in quenched martensite. There is nofinal conclusion about the strengthening form of the carbon atoms in quenched martensite.The martensite structure is complex, and is divided into packet, block and lath by thecrystallography orientations. Conclusions about the microstructural unit controlling themartensite strength are inconsistent. Viewpoints regarding prior austenite, packet andblock as the microstructural unit controlling the yield strength all exits. However, mostarticles considered microstructural unit controlling the yield strength, and didn’t study themicrostructural unit controlling the ultimate tensile strength. The medium carbonquenched martensite steel is brittle due to the large internal stress and the appearance oftwins. It is one of the martensite study content at all times that how to improve theductility and toughness to fully utilize the high strength of the medium carbon martensitesteel.This paper quantitatively characterized the microstructure of the quenchedmedium-Mn martensitic steels using various experimental means, on the base of whichdiscussed the carbon strengthening form in quenched martensite and the microstructuralunit controlling the strength. The quenched medium carbon medium-Mn steel is verybrittle. It was deemed that the brittleness is related to relatively high content of unstableretained austenite (14%, volume fraction) in the microstructure. The ductility of thequenched medium carbon medium-Mn martensite steel was effectively improved throughcontrolling the the content and stability of the retained austenite by conventional tempering process. The quenched martensite strength is very high, but the ductility andtoughness is low. To improve the comprehensive mechanical properties, the low carbonmedium-Mn martenise steel was tempered, and the tempering temperature effect on themicrostructure and mechanical properties was studied.The study on the quenched medium-Mn martensite steels indicated that, increasingcarbon content increase the dislocation density and refine the packet and block size. Grainrefinement and dislocation density increase made the increase of the yield strength andultimate tensile strength, while the ductility lowered gradually. Calculations proved that,carbon in quenched martensite doesn’t act solid solution like strengthening effect. Thecarbon effect can be understood as enhancing the ability of dislocation strengthening andgrain refinement strengthening. The microstructural unit controlling the yield strength andultimate tensile strength is prior austenite and lath, respectively.The study on medium carbon medium-Mn steel (Fe-0.4C-5Mn) indicated that,ultrahigh strength steel with good ductility was obtained by low temperature tempering,with ultimate tensile strength of above1600MPa and ductility of12%~15%. The reason ofits ultrahigh strength is martensite matrix strengthening; Meanwhile, the high volumefraction of retained austenite with high stability favors the good ductility.The study on the low carbon medium-Mn steel (Fe-0.2C-5Mn) indicated that,tempering heat treatment improve the ductility and toughness of the martensite steel. Lowtemperature tempering when tempered at200℃obtained the best comprehensivemechanical properties with the ultimate tensile strength of1600MPa, yield strength ofabout1250MPa, elongation of15%and room temperature Charpy-V impact energy of28J.The reason of the tempered martensite embrittlement when tempered at about300℃is thedecomposition of the inter-lath retained austenite. |
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