Study On Cu Precipitate Of High Strength Low Alloy Steel Containing Cu And Ni During Tempering

High-strength Low-alloy (HSLA) steel is mainly used on structuralmachines, submarine or ship. HSLA steel is famous for its high strength andexcellent toughness. The design philosophy is to reduce C content to0.05wt.%to increase toughness and weldability, add Cu content to1-2wt.%to increasestrength via precipitation hardening effects. Cu has a low solubility in steel andis added to provide an increased strength by precipitation of nanoscale Cuprecipitates during tempering. In the study, the specimen was used as ship boardsteel. These specimens were austenitized at900℃for30min followed by waterquenching, then tempered at500℃for different period or at differenttemperatures for60min. Specimens after heattreatment were analysed by atomprobe tomography (APT) and high resolution transmission electron micrography(HRTEM). The main conclusions were as follows:（1） During tempering, matrix recovered and steel was strengthened bynanoscale Cu precipitates; Lath boundary gradually bulged out andmigrated, a repeat of bulging and migration of local parts of lathboundary resulted in migration of the whole boundary, and lathmartensite transformed to equiaxed ferrite finally.（2） The number of Cu clusters changed differently as a function of Cuisoconcentration value when Cu precipitates were under different stages;When Cu precipitates were on the stage of nucleation, the number of Cuclusters changed greatly; When Cu precipitates were on the stage ofcoarsening, the number of Cu clusters kept a constant.（3） APT analysis showed that the number density of Cu clusters was at themagnitude of1024/m3; Ni and Mn segregated at the heterophase betweenCu precipitate and matrix forming a core-shell structure; the peak concentration of Mn is close to Cu core compared with that of Ni.（4） At the heat treatmet of400℃-1h, grain boundary provided preferentialnucleation sites for Cu precipitation, and atoms C, Mn, P and Cusegregated at the grain boundary; at the heat treatmet of500℃-4h, Cuclusters and atoms Cr, Mo and C segregated at the grain boundary; AtomsC and Mo segregated along dislocation line; Cu cluster alternatered withC cluster to precipitate at the dislocation.（5） HRTEM showed that Cu precipitates of9R or fcc structure precipitated,and Cu precipitate Moiré fringe formed; Digital Micrograph analysisshowed that there existed a Cu precipitate of9R structure bigger than thatof fcc structure, this research indicated that there was not connectionbeween Cu precipitate structure and Cu precipitate size.