The Impact On The Microstructure And Properties Of Low Alloy High Strength Steel By Adding Calcium And Magnesium

The rapid development of large steel structure has brought great opportunities to the steelmanufacturing industry, but as the requirement of design became harder and themanufacturing technology of large steel structure advanced, and the requirement on theperformance of the steel plate also has grown harder. Not only the fundamental performancelike the strength and toughness was necessary, but also the steel has to be capable of enduringlarge heat input during welding. Selecting oxides particles with high melting point and highstability to pin the grain boundary, which can restrain the crystal grain from grow big, hasbecome a good way to effectively improve the weldibility of the steel structure. Moreovercombining the controlled roll and controlled cooling process, the low alloy high strength steelwith favorable strength and toughness, also with excellent weldibilty by high heat input, canbe obtained.In this paper, we adopted metallargy thermal dynamic theory, metallary physics chemitrytheory and material formation control methods, also with optical microscope, SEM EDX andmachanical anlynasis methods, we studied the adding elements of Ca and Mg technology thatcan ensure to form the stable, super thin, uniformly distrbuted,and composition controllablehigh melting point inclusions，in high tempreture.and we studied the formation and rollcontrol technology of the Ca/Mg low alloy high strengh steel. Systematicly studied the impactof Ca and Mg to the microstructure, inclusion morphology, compositon, distribution andmachanical performance of the low alloy high strengh steel,claimed the mechanism of howthe Ca and Mg improved the strengh and toughness of the low alloy and high strengh steel.In this experiment we meltied out the high-strength low-alloy steel designed by our ownin vacuum induction furnace, using a special process that respectively added different amountof calcium, magnesium to the steel. By controlled rolling and cooling process, which rolled and cooled the experimental steel after austenitizing, and found that the microstructure of theexperimental steel is mainly constituted by the polygonal ferrite, a small amount of pearliteand bainite. The addition of calcium and magnesium caused significant effect on themicrostructure and properties of the raw steel. The grain size was refined, the content of smallinclusions increased and more evenly distributed, the strength and toughness was significantlyimproved, in the process the fine grain strengthening and precipitation strengthening are themajor strengthening mechanism. Compared with the raw material of steel, adding5%the Mgelement to the experimental steel refined the grain size by48%; the quantity of the producedsmall inclusions grew, most of the inclusions consisted of MgO-ZrO2-MnO, and uniformlydistributed in the steel; the increment of tensile strength and yield strength were155MPa and205MPa; impact toughness increased nearly by three times, up to253J; the impact fractureswere made of dimple fracture patterns.