Flattening Of The Austenite Phase Representation And The Development Of Steel With High Strength And High Tenacity

Low (Ultra-low) carbon bainitic steel has good properties and gets more and more attention. Mcrostructure and mechanical properties in steels at toom temperature are closely related to the austenitic structure at high temperatures. There are several factors which affect the austenitic structure at high temperatures, such as alloying elements, the heating temperature, deformation at high temperatures.In order to research the effects of austenite deformation before transformation on the transformation of multiscal structure. The evolution of austenite grain shape and intergranular interface in Ni-30Fe model alloy during different reduction, Austenite became pancaked during the deformation, the density of austenite grain boundary and transgranular interface increased. Then research the austenitic structure and the multiscal structure of0.06C-2.9Mn ultra-low carbon martensite steel during diferent reduction before transformation. With the deformation increasing, the austenite is strengthened. Intergranular block size is fined by the strengthened austenite, then the strength of ultra-low carbon martensite steel is improved.A new type ultra-low carbon bainitic steel with high toughness was developed based on the novel chemical composition design, which consists of the alloying of Mn, ultra-low carbon and micro-alloying with Nb and Ti. Through the thermo-mechanical processing, refined bainitic micrstructure formed from pancaked austenite. The results show that the average thickness of pancaked prior-austenite is reduced to about4μm in a0.03%C-3.4%Mn micro-alloyed steel, which resulted in a good combination of yield strength is600MPa and tensile strength is780MPa, and impact energy of the upper shelf is230J. The microstructure of transformed product in the steel studied is characterized using electron backscattered diffraction (EBSD) technique. In addition, the strengthening and toughening mechanisms and the advantages of the composition and processing design were preliminarily analyzed.With total compression ratio of10, the industrialized trial of the third high toughness Q890steel is taken. Pancaked austenite is obtained through the TMCP process, the average thickness of pancaked prior-austenite is about5μm, the density of the interfaces are increase after martensite (bainite) transformation, which resulted in a good combination of strength and toughness. The yield strength is900MPa and tensile strength is1040MPa, and-40℃impact energy is above200J, ductile to brittle transition temperature is down to-80℃below. This steel breaks through the same level of intensity micro-alloy steels which are produced by QT or TMCP+tempering process, the cost is reduced.