Nano Cu Precipitation And Retained Austenite Design And Mechanical Behavior Of Al-bearing Low Alloy Steels

Designing the M3(multi-phase,meta-stable,and multi-scale)microstructure is of great significance for the development of high-performance engineering steels.Based on the alloy design and process controlling,this work designed the multi-phase,in low-C low-Mn steels.Utilizing the precipitation strengthening effect of Cu particles in the multi-phase microstructure and the transformation-induced plasticity(TRIP)of retained austenite,high-strength high-ductility steels have been developed.To achieve the optimal precipitation strengthening effect through copious nano Cu preciptation,the influence of Al addition on the precipitation behavior of Cu in low-C low-alloy steels was studied.It was found that the addition of 1 wt.%Al significantly increased the number density of Cu-rich particles from 6.2±0.7×1023 m-3 to 13.9±0.9×1023 m-3,while decreased the average size of Cu-richparticles on grain boundaries and dislocations from 2.4±0.5 nm to 1.9±0.5 nm.After tempering for 2 h,the significant enrichment of Al in Cu-rich particles was observed.To eliminate the potential interactions between multiple alloying elements and Al which may influence the Cu-rich precipitation,the effects of Al addition on precipitation behavior at early stages of tempering in Fe-Cu and Fe-Cu-Mn alloys were further studied.The results showed that after tempering at 550? for 5 min,the addition of 1 wt.%Al increased the number density of Cu-rich precipitation in Fe-Cu and Fe-Cu-Mn alloys by 7 and 2 times,and decreased the average radius by 57%and 42%,respectively.The Al addition was suggested to distinctly increased the number density and reduced the size of Cu-rich precipitates,while the Mn addition weakens the beneficial effects.The key for the synergistic effects of precipitation strengthening and transformation-induced plasticity(TRIP)is to obtain sufficient amount of stable retained austenite during nano Cu-rich precipitation formation process.The quenching&partitioning-tempering(Q&P-T)treatment was carried out in a low-C low-Mn Al-Cu bearing steel,Q&P-T sample obtained a high density of Cu-rich particles(1.22×1024 m-3)and 20 vol.%carbon-enriched retained austenite.Comparing with the Q&P steels,the tensile strength and yield strength were increased by 130 MPa and 297 MPa,and the total and uniform elongation were simultaneously increased by 3%and 4%,respectively.It was found that the Q&P samples are composed of retained austenite,carbon-enriched fresh martensite and tempered martensite.During further tempering,carbon were partitioned from fresh martensite into the retained austenite which improved the stability of retained austenite,and the nano Cu-rich precipitation increased yield strength.That is why the strength and ductility simultaneously increased in the Al-Cu-bearing low-C low-Mn steel.To simplify the Q&P-T process and obtain stable retained austenite by C and Mn,the intercritical annealing and tempering(LT)process was designed for the Al-Cu bearing low-C low-Mn steel,and the influence of hot/cold rolled deformation microstructure on the microstructure evolution and mechanical properties during LT was studied.The results showed that during intercritical annealing process,for the hot-rolled samples,the lath-shaped reverse austenite was formed and almost all retained to room temperature;For the cold-rolled samples,the globular reverse austenite was formed with larger size and lower stability,and half of them transformed into fresh martensite in which carbon was riched.During the subsequent tempering process,the majority of lath-shaped retained austenite in the hot-rolled samples was decomposed,while most of the retained austenite in the cold-rolled samples was stable due to the partitioning of carbon between fresh martensite and retained austenite,which increased the stability of austenite,thus increasing the ductility.During tempering,the Cu-rich precipitation formation increased the yield strength by 150?200 MPa.