Study On Improving The Tensile Properties Of Nano-phase Strengthened Ferritic Steel Aged In Air

High toughness alloy steel is mainly divided into martensitic steel and ferritic steel.Brittle HAZ easily occurs in martensitic steel due to its high content of carbon after welding.The ferritic steel can own excellent comprehensive mechanical properties through the formation of nano-precipitation strengthening phase on its matrix by means of solid solution and aging.However,it is easy to produce brittle fracture for nano-phase strengthened ferritic steel after air aging.Therefore,Y element is added in ferritic steel with the composition of Fe-Cu-Ni-Mn-Al,and the deformation process,solid solution and aging,and tensile properties test are compared with the steel without Y addition so as to explore the mechanism of improving the tensile properties after air aging for Cu-rich nano-phase strengthened ferritic steel.The microstructure,tensile fracture and oxide layer of the samples were observed by optical microscope(OM)and scanning electron microscope(SEM).High resolution transmission electron microscopy(HRTEM)and energy dispersive X-ray spectroscopy(EDS)were used to analyze the morphology,size,number density,chemical compositions and crystal structure for nano-crystalline precipitates.The following results were attained:(1)The optimal preparation process is the process ?,i.e.the process of " solid solution?water cooling?air aging?water cooling”or“billet?heated to 900 ? for 30 min?water to room temperature?removing surface oxide layers?air heated to 550? for 17 h?water to room temperature" in detail.(2)Compared with the sample without Y addition,precipitation density was decreased,precipitates were coarsened,the distance between precipitates was increased,matrix grains were refined,and the strength and ductility were improved for the sample with Y addition.(3)The surface oxide layers became thinner,the oxide layer/oxide layer,the oxide layer/the matrix were firmly combined,and the oxidation resistance was improved obviously for samples with Y addition.(4)A large number of Cu-rich nano-phases precipitated in steel matrix.The precipitation process of Cu-rich phase was bcc?9R ?fcc.(5)Not only the Cu-rich phase precipitated,but also the Y-rich phase precipitated in the matrix of air aged samples with Y addition.The Y-rich phases existed mainly in the form of YAs,Ni5Y,Y2As4Cu2 and Y5S7.Besides the nano-sized precipitates in ferritic matrix,there were also a small number of micron-sized Y-rich precipitates containing S and Se,ranging from 1 to 2 ?m.The improvement in brittle fracture for nano-phase strengthened ferritic steel after air aging is due to the grain refinement,antioxidation and purification from Y addition...