Hot Deformation Behavior And Microstructural Evolution Of Duplex Stainless Steels

The microstructure of duplex stainless steels(DSSs)is composed of austenite and ferrite phase after solution treatment,while each phase proportion is about 45%-55%.Because it possesses excellent corrosion resistance,well mechanical properties and weldability resistance,duplex stainless steels can be widely applied to industrial fields.e.g.pulp.paper making,marine transportation,petroleum chemistry,natural gas and so on.However,the flow behavior of DSSs is more complex than those of the austenitic and ferritic stainless steels due to differences in strength,flow behavior and softening mechanism between each constituent phase.Nowadays,it is difficult to produce plate and large diameter thick-walled pipe of DSSs because of some technique problems.Therefore,this paper choosed DSS2205 and super duplex stainless steel SDSS2507 as the object,so as to comprehensively investigate flow behavior and fracture mechanism of DSSs during hot deformation,while mechanical property and hot rolling process parameters were also researched.In this paper,the following results can be obtained:1.Sigma phase is the most harmful intermetallic compounds in stainless steels.It was found that,the precipitation rate of ? phase in SDSS2507 had been obviously accelerated and the transformation mechanism was also more complex by comparison to DSS2205.During isothermal aging at 850?,it precipitated via mechanisms of ???+?2 and ???,respectively.Based on two different precipitation behaviors of sigma phase in hot rolling SDSS2507,the JMA model was modified and a good self-consistent result could be obtained.2.Flow behavior and strain partition in each constituent phase of DSSs during hot compression was studied.Constitutive equations and processing maps were also established.The dynamic softening of austenite and ferrite had also been revealed.It can be found that,the dynamic softening mechanism in ferrite phase was dynamic recovery and continuous dynamic recrystallization,and that in austenite corresponded to discontinuous dynamic recrystallization.Temperature and strain rate were the main factors of strain partition and hot deformation activation energy of DSSs.At a lower strain rate(0.01s-1),the flow behavior of DSSs was controlled by the softer ferrite phase,causing that hot deformation activation energy declined continuously during hot compression.By comparison,the dynamic recrystallization of ferrite was postponed when the strain rate increased.while the plastic deformation in austenite was enhanced,which resulted in platforms in deformation activation energy curves at the intermediate deformation stage.Processing maps of DSSs proposed that,the lower strain rate and higher temperature will increase the flow stability of DSSs and improve the hot plasticity of the alloys.3.Hot deformation behaviors in hot tensile and torsion tests were investigated.respectively.In hot tensile,the hot ductility of DSS2205 was mainly affected by the softening capacity of each constituent phase,and the reduction of area of the alloy remained nearly 90%within 1050-1200 ?;In torsion test,the distribution of softer ferrite controlled by the phase boundary became the controlling factor of the hot plasticity,which caused that the hot plasticity exihibited more temperature-sensitive within 950-1200?.Based on the analysis,this paper proposed that,the optimization in distribution of two phase boundaries should be the effective approach to solve the problem of low yield in the production of large diameter seamless steel pipe of DSSs.4.Torsional deformation behavior of the alloy was further studied by High Pressure Torsion(HPT),while the relationship between the microstructures and mechanical properties has also been investigated.It can be found that,the effect of phase boundary on the mechanical properties of DSS2205 was significant.Accordingly,the relationship between average width of two phases and hardness of DSS2205 conformed to the Hall-Petch relationship.During HPT,the ferrite grains were refined via the interaction of deformation dislocations.The grain refinement in austenite was more complex and mainly dependent on a mixed mechanism including deformation twinning and interaction of dislocations and twin boundaries at the early deformation stage.At the later stage of deformation,the refinement in austenite significantly hindered the mechanism of deformation twinning while dislocation glide was the dominant deformation behavior.5.Double-pass compression has been preformed with DSS2205 and SDSS2507,and microstructural evolution of DSSs during the interval of hot deformation has been studied.It can be found that,the extended holding time at 1200? simultaneously increased the static softening rate of the alloy and decreased the deformation resistance during second pass deformation,which was beneficial to the subsequent hot processing of DSSs.In a lower temperature,a large number of the secondary y phase formed in ferrite,which was more obvious in DSS2205 than SDSS2507 and induced the increase of the deformation resistance in the second pass deformation.6.To solve the problem of surface and edge cracks in hot rolling plates,the rolling process parameters which can significantly improve the surface quality of DSS2205 were presented.It was found that,the effect of deformation zone shape parameter on the microstructural and plastic deformation uniformity of the alloy was very significant.In addition,which can be obtained in the multi-pass hot rolling of DSSs was that,the reasonable design of deformation zone shape parameter ? in each pass rolling can improve the plastic deformation uniformity and the surface quality of hot rolling plate.