Effects Of Heat Treatment And Hot Processing On Banded Structure Characteristics And Mechanical Properties Of PCrNi₃MoV Steel

PCr Ni₃Mo V steel was developed based on the Cr-Ni-Mo high strength steels by V micro-alloying.Due to the excellent hardenability,high strength and good toughness,PCr Ni₃Mo V steels have been widely used as structural components for gun barrel and ultra-high pressure vessels.Hardenability and mechanical properties of steels can be further improved by the addition of alloying elements.However,segregation of alloying elements during smelting and forging will induce the formation of banded structure.As inhomogeneous microstructure in steels,banded structure can lead to delaminated structure,changing structural states of steels.Upon tensile loading,the delaminated structure dominates the initiation and propagation of crack,thus affecting the mechanical behavior of steels.Previous studies on banded structure usually focused on dual-phase steels,and studies on banded structure in single-phase steels are seldom reported.The formation mechanism of banded structure in Cr-Ni-Mo-V steels is still not clear,as well as microstructure evolution of banded structure upon heat treatment and hot deformation process.Effects of lamellar structure induced by banded structure on mechanical properties of PCr Ni3Mo V steel is also not clear.In this study,effects of various heat treatments and hot deformation on microstructure evolution of banded structure in PCr Ni3Mo V steel was systematically studied.The relationship between element segregation and banded structured prior austenite grains is established;effects of quenching temperature on inhomogeneous growth of banded structured grains are revealed;precipitation behaviors of carbide within banded structure upon tempering treatment are clarified;mechanical characteristics of different areas in banded structure are analyzed and effects of banded structure on mechanical performance of PCr Ni3Mo V steel was discussed.The main contents and results are as follows::(1)The banded structure in as-quenched PCr Ni₃Mo V steel is characterized by alternately distributed fine and coarse prior austenite grains.The formation of banded structure is suggested to be related to the element segregation of C and alloying elements Mn,Cr,Mo and V in continuous casting,and the prior austenite grains in element-enriched area exhibit smaller sizes.During austenitizing process,the growth of prior austenite grain in element-enriched area is not obvious,while that in element-poor area is considerably obvious.With the increase of quenching temperature,the tensile strength,ductility and toughness of PCr Ni₃Mo V steel are increased.The dislocation accumulation caused by the uneven deformation at the interface between individual layers is the main reason for the increased strength.Cracks tend to occur at the delamination interfaces and then propagate along the interface,resulting in delamination cracks.The crack blunting of main crack by deflecting its propagation at interface is the main reason for plasticity and toughness improvement.(2)Tempering treatment has a negligible effect on the morphology and distribution of banded structures in PCr Ni3Mo V steel.However,carbide precipitation in different individual layers of banded structure exhibits different characteristics.More carbides are precipitated within fine-grained area than within coarse-grained area,and the amount of retained austenite in fine-grained area is higher.Tempering temperature has a significant effect on types and distribution of carbides in PCr Ni3Mo V steels.At the tempering temperature of 430/580/630°C,the carbides in PCr Ni₃Mo V steel are MC/M₃C,M₂C/M₂₃C₆ and M₂C/M₇C₃ respectively,and the MC carbides exist stably in the tempering microstructure.With the tempering temperature increasing,the strength and hardness of PCr Ni3Mo V steels are decreased,while the ductility and toughness are improved.There is an evident secondary hardening at 580°C.The delaminated layers on the fracture surface are suggested to be related to the banded structure.Upon tensile loading,the stress state transforms from plain strain state to plain stress state,leading to delaminated fracture surface.The change in stress state can improve the energy release during crack growth,which is beneficial to toughness improvement.(3)Hot deformation behaviors of PCr Ni3Mo V steel are investigated by isothermal compression at temperature range of 900?1200°C with strain rates from 0.01 to 10 s^-1.The constitutive equation is derived on the basis of true stress-true strain curves and the processing maps are also established.Effects of hot deformation conditions on microstructure evolution of banded structure are discussed.When the steel is deformed at strain of 0.6,with strain rates of0.01?1 s^-1 and deformation temperature of 900?1200°C,the banded structure is nearly eliminated.At deformation temperature of 900?1000°C and strain rate of 10 s^-1,banded structure can still be found.With the deformation temperature increasing from 1100°C to1200°C,the banded structure is gradually vanished.