| As the rapid development in the global economy,the heavy-loading bridge,high performance great ship and so on have necessitated the steels with superior performances,i.e.light weight together with high performance.Accordingly,it is very important to develop new steels with both high strength and good plasticity.In this study,the 25Mn2 SiCr and 25Mn2 SiCrV steels as starting materials were used and the effect of chemical compositions and heat treatment on their microstructures and mechanical properties through one-step and two-step quchening-partitioning(QP)process were investigated.We obtained the following results:1.The amount of retained austenite in the 25Mn2 SiCr steel initially increased and then decreased with an increase in partitioning temperature,and reached its maximum of 7.0% at 230℃ during the one-step QP process when holding at different temperatures for 5 min after austenitizing at 910℃.The two-step QP process,i.e.quenching at 230℃ and partitioning at 400℃,increased its amount of retained austenite.And the sample quenched at 230℃ for 30 s had the maximum retained austenite of 10.4%.Increas ing in partitioning temperature decreased the retained austenite.The introduction of vandium in the 25Mn2 SiCr steel increased its partitioning temperature and the maximum amount of retained austenite,i.e.7.7% for one-step QP process when partitioning at 260℃ and 12.3% for two-step QP process when quenching at 260℃ for 90 s and then partitioning at 400℃ for 90 s,respectively.2.With an increase in the partitioning temperature,strength of the studied steels decreased and their plasticity initially increased and then decreased during the one-step QP process.The 25Mn2 SiCr steel presented the optimum mechanical properties such as tensile strength of 1688 MPa,yielding strength of 1205 MPa,elongation of 17.2% and shrinkage of 52.0%.Its product of tensile strength and elongation was about 29.0 GPa·%,this value was about 44% higher than that of conventional quenching and tempering process,and about 46% higher than that of forged one.On the other hand,the V-containing 25Mn2 SiCrV steel obtained its optimum mechanical properties when partitioning at 260℃,such as tensile strength of 1687 MPa,yielding strength of 1224 MPa,elongation of 18.3% and shrinkage of 53.4%.Its product of tensile strength and elongation was about 30.9 GPa·%,this value was about 6% higher than that of the steel without vandium.3.With an increase in the partitioning time,strength of the studied steels decreased and their plasticity simultaneously increased during the two-step QP process.However,all their mechanical properties decreased when the partitioning temperature increased.The 25Mn2 SiCr steel presented the optimum mechanical properties such as tensile strength of 1675 MPa,yielding strength of 1201 MPa,elongation of 19.5% and shrinkage of 54.0%.Its product of tensile strength and elongation was about 32.7 GPa·%,this value was about 12% higher than that of one-step QP process.On the other hand,the V-containing 25Mn2 SiCrV steel obtained its optimum mechanical properties when quenching at 260℃ for 90 s and partitioning at 400℃ for 90 s,such as tensile strength of 1669 MPa,yielding strength of 1256 MPa,elongation of 20.1% and shrinkage of 55.1%.Its product of tensile strength and elongation was about 33.6 GPa·%,this value was about 3% higher than that of the steel without vandium.4.Microstructure observation revealed that both of the steels consisted of typical lath martensite and retained austenite,which existed between the lath martensites in the form of films.The retained austenite during one-step QP process was thinner than that of two-step one.Addition of vanadium in the 25Mn2 SiCr steel couldn’t change its microstructure,but the length and packet of the martensite laths became smaller and retained austenite between the martensite laths increased. |
