The Effects Of Alloy Elements On Microstructures Of Low-alloy Ferritic Steel

Four steels with varying manganese,nicknel and copper contents were used in this study.After an initial heat treatment of 0.5 h at 880℃.Then water quenching followed by at 660℃for 10 h and air-cooled.Finally,the Samples were then isothermally aged at370℃for different times up to 4000 h.The Vickers microhardness was messured by microhardness tester with a load for 100 gf for 15 s.The microstructures of Cu-bearing ferrite steel were investigated by metallographic microscope,SEM,TEM,HRTEM,APT and EDS.The main conclusions are described as follows:（1）Cu refine ferrite grain and strengthen the matrix in Cu-bearing ferrite steel.Ni improve hardenability of steel and stability of retained austenite film.The increasing of Cu and Ni content in Cu-bearing ferrite steel can promote the precipitation of carbide and nano size precipitation,which significantly improve the hardness of steel.（2）It is found that the granular bainite formation in Cu-bearing ferrite steel is attributed to a mixed mode of diffusional and displacive mechanisms.Besides,the particle-like or plate shapedωphases,along with thin-plate martensites consist of twin,are observed in the high Mn steel,and the orientation relationship betweenωphases and thin-plate martensites is determined as:[2-1-10]ω//[011]αand（01-10）ω//（2-11）α.The ultra-fine or ellipsoidal shaped nanoscale orthorhombic martensite phases,along with laths martensites consist of a high density of dislocations,are observed in the low Mn steel,and the orientation relationship between orthorhombic martensite phases and laths martensites is determined as:[01-1]o//[-111]αand（-111）o//（101）α.A model is proposed to describe the overall martensite transformation kinetics by taking into account the Mn content,heterogeneity of C content and dislocation reactions play a major role in the subsequent martensite transformation.（3）The residual austenite film of nano size distribution between lath martensite or bainite ferrite.The retained austenite film between lath bainite ferrite is more thicker than between lath martensite.A Nishiyama-Wassermann relationship between martensite and retained austenite film:[0 0 1]α’//[0 1 1]γand（1 1 0）α’//[-1-1 1]γ.Theθ-NiMn intermetallic compound has a face centered tetragonal crystal structure which is almost identical to the complex face centered cubic crystal structure of the M₂₃C₆ carbides.（4）After aging 3000 h,the Cu-rich clusters of two kinds of morphologies,ellipsoidal containing high Ni content and short rod-shaped containg high Mn content,were observed using APT.During the growth of the Cu-rich clusters,the Mn atoms in the Cu-rich clusters are preferentially ejected from the clusters.In addition,the Cu-rich clusters are preferentially precipitated and grown in the interface of the cementite.The driving force of the nucleation and growth was proposed by the energy fluctuation in the interface.The number density of Cu-rich clusters increased with aging times up to 4000 h,which indicated that the growth of Cu-rich clusters was promoted with aging time prolonged.