含Cu和Ni低碳高强度钢等时回火析出富Cu相的研究

STUDY ON Cu PRECIPITATE OF THE LOW C HIGH STRENGTH STEEL CONTAINING Cu AND Ni DURING ISOCHRONAL TEMPERING

用显微硬度计、HRTEM和三维原子探针(31AP)对含Cu和Ni低碳高强度钢等时回火析出的富Cu相进行了研究.结果表明:回火过程中,基体发生软化,富Cu相析出,板条状马氏体逐渐转变成多边形状铁素体;在500℃时富Cu相强化作用达到最大值;设置不同的Cu等浓度值时,在400-500℃富Cu相的数量变化幅度大,在500-650℃富Cu相的数量基本不变;在晶界处发生C,Mo,P和Cu的偏聚;晶界处Cu浓度高于基体,为富Cu相的形核和长大提供了有利条件;在析出的富Cu相与基体的过渡层上发生Ni,Mn和Al的偏聚,这些偏聚元素与富Cu相核心共同形成核-壳结构.

Cu precipitation strengthening plays an important role in the fabrication of high-strength low-alloy （HSLA） steels. The nature of Cu precipitation and the actual distributional morphology of Cu precipitates have a significant effect or directly determine the strength and toughness of HSLA steels. HSLA steel is weldable without preheat by reducing C to a low concentration. To compensate for the decrease of strength caused by reducing C, Cu was added to HSLA steel for precipitation strengthening by nanoscale Cu precipitates. The size, number density and composition of Cu nanophases could be well characterized by the atom probe tomography （APT）, and the Cu nanophases obtained by APT analysis are usually termed Cu clusters. In the study, the specimens were austenitized for 30 min at 900 ℃ followed by water quenching, and tempered isochronally for 60 min at different temperatures. The hardness was conducted, the microstructure and Cu precipitate were analyzed by HRTEM and APT. During tempering, Cu precipitation happened, Cu precipitate Moir~ fringe formed and the Cu precipitate transformed to fcc structure; the lath boundary gradually bulged out and migrated, a repeat of bulging and migration of local parts of lath boundary resulted in migration of the whole boundary, and lath martensite transformed to equiaxed ferrite finally. At 500℃, the strengthening peaked by Cu precipitates. During 400--500℃, the number of Cu clusters changed greatly when the Cu isoconcentration set at different values, this indicated that the Cu precipitates were on the stage of nucleation; while the number of Cu clusters changed little during 500--650℃, this indicated that the Cu precipitates were on the stage of coarsening. The Cu, C, Mo and P segregated at the grain boundary. The boundary could provide Cu solutes and nucleation sites for Cu precipitation, leading to the segregation of Cu clusters at the grain boundary. The Ni, Mn and Al segregated at the heterolohase interface between Cu precipitate and ferrite matrix forming a core-shell structure.