Effect Of Tempering Process On Microstructures And Mechanical Properties Of Low-carbon High Strength Bainitic Steel

Low-carbon high strength bainitic steel is recognized as an important kind of steel in21st century. Modern steel-making process is used for remelting this type of steel to make its carbon content very low and to possess very excellent welding performance. Since the carbon content is reduced in an extremely low level, the traditional upper and lower bainite have also been changed to low-carbon or carbon-free bainites and the toughness is also very good. Different from the traditional high-strength steel, the microstructures and mechanical properties of this kind of steel is strongly influenced by tempering process. So, there is a need to do some tempering experiments on this steel to study the microstructural evolution and mechanical properties. The aim of this study is to have a deep understanding of the dependence of tempering process on the microstructures and mechanical properties such that a suitable tempering process can be obtained in industrial production.In this thesis, TMCP technology was used to roll the low-carbon high strength bainitic steel plate. A series of tempering processes was then designed and various means for characterizing microstructures and measuring mechanical properties were used to analyze the variation of microstructure and mechanical properties of this steel with tempering. The following research results could be obtained.(1) After rolled by TMCP technology, the microstructure of this steel is mainly granular bainite and lath bainite surrounded by some fine M-A islands. The tensile strength and impact toughness of the as-rolled steel are higher. The main strengthening mechanisms of this steel are grain refinement, solid solution strengthening, M-A island strengthening, dislocation strengthening and precipitation strengthening.(2) The microstructures of this steel after tempering were characterized by various microstructural analyzing means. The results show that quasi-polygonal ferrite is most stable in the microstructures after tempering. The second is granular bainite and third is lath bainite. The residual austenite distributed between bainite was decomposed first during the tempering process. As the tempering temperature is increased, quasi-polygonal ferrite is the final microstructure of test steel and it is equilibrium state.(3) By observation under TEM, we can find that when tempered at above350℃carbonitrides of Nb, Ti, V begin to precipitate in this steel. With the tempering temperature and time are increased, the amount of precipitation is increased. At600℃×1.5h, the amount of precipitates reach peak. When tempered at above650℃, the precipitates begin to accumulate and grow up.(4) When the steel is tempered at600℃for1.5h after rolling, a comprehensive excellent mechanical properties can be obtained. In this case, the tensile strength is897.5MPa, similar to that of rolled state. Yield strength is the highest and reaches775MPa, increased by175MPa as compared with that of rolled state. However, its impact energy and hardness value are somewhat lowered to146J and294HV, respectively, as compared with rolled state.