Influence Of Mn On High Temperature Tensile Behavior Of 18%Cr-node Nickel Duplex Stainless Steel

Duplex stainless steel(DSS)is composed of two-phases balanced structure of austenite and ferrite,which is widely used in paper industry,nuclear power,marine engineering and construction as a high performed structural material due to its excellent corrosion resistance,weldability and high strength.In order to keep the balance of two phase,the use of cheaper austenitic stabilizing Mn can replace most part of expensive Ni element to stabilize the austenite phase,and can also improve the solubility of N in the steel.There are different crystal structure and stacking fault energy(SFE)for two phases of DSS,and the Mn substitution for Ni also can change the value of SFE for two phases and cause austenite stability variation during high temperature deformation.Thus,the Mn addition will affect the deformat ion mechanism of the two phases,which makes their hot deformation behavior more complex compared with that of single phase steel during high temperature stretching.In this paper,the experimental steels with different Mn content gradients were added and compared with austenitic stainless steel with the same content of Cr,and the effect of Mn contents on the high-temperature tensile mechanical properties,thermoplasticity and evolution of submicroscopic structures for tested DSS were studied by using a thermal simulation machine.These studies can provide theoretical basis for high-temperature processing and mechanical properties,and has great significance for guiding its drawing production process.The main conclusions are as follows:1.The work hardening rate,peak stress and peak strain decreased with the increase of deformation temperatures for different Mn addition samples.The microstructures of two phases near the fracture of different Mn addition samples are refined to some certain extent,and the ferrite phase near the fracture becomes slender and the grains become smaller.Compared with3.1%Mn and 9.0%Mn experimental steels,the ferrite phase of 5.8%Mn experimental steel is more slender,and the grain size of it become smaller.2.Under the tensile conditions of 0.005 s^-1 and 0.05 s^-1,more Mn addition has little effect on the work hardening rate deformed at 350??500?,while the increase of Mn content deformed at 650??950?has an influence on the work hardening rate,but the dynamic recrystallization(DRX)occurred at deformation temperature of 1050?for different Mn addition samples.Increasing Mn content from 3.1%to 5.8%effectively decrease the peak strain,but the addition of 9.0%Mn slightly increase its peak strain,which is not favorable for DRX occurence deformed at lower strain.3.At the same strain rate(0.005 s^-1,0.05 s^-1),the elongation of different Mn content steels decreased greatly with deformation temperatures at 350??800?,but increased at higher deformation temperature 800?.At the same deformation temperature,the elongation of the experimental steel with different Mn content decreases with the increase of strain rate.For 3.1%Mn?9.0%Mn addition samples,the formation of?precipitates reduced plasticity at 800?with the strain rate of 0.005 s^-1 and 0.05 s^-1.Under the same deformation conditions,the best plasticity was obtained for the sample with 5.8%Mn addition steel due to its highest compared with other Mn addition samples,but the elongation with higher Mn addition of 9.0%decrease.The elongation of Mn-3.4Ni austenitic stainless steel at lower deformation temperature(350??800?)is higher than 3.1%Mn experimental steel but lower than 5.8%and 9.0%Mn addition samples.4.When stretched at the strain rate of 0.005 and 0.05 s^-1,the tensile strength of different Mn content steels decreased obviously with the increase of deformation temperature at low deformation temperature of 350??500?,but decreased less at higher deformation temperature(650??1050?).At the same deformation temperature,The tensile strength of3.1%Mn and 5.8%Mn experimental steel increase when the strain rate increases from 0.005 s^-1to 0.05 s^-1,but it has little effect on the tensile strength of 9.0%Mn experimental steel and 0.08%Mn-3.4%Ni experimental steel.Under the same deformation condition,the increase of Mn addition can effectively improve the tensile strength of experimental steels for 3.1%Mn?5.8%Mn,but the increase of 9.0%Mn content will lead to a certain reduction of the tensile strength.5.The fracture morphology of different Mn content steels are ductile fracture with good plasticity.The dimples became deeper and uniformer when the Mn content increases from 3.1%Mn to 9.0%Mn.While the dimples and second phase particles of high 9.0%Mn addition steel were larger,and the plasticity decreased to a certain extent.The second phase particles are mainly((Cr,Mn)O)composite oxides in the different Mn content steels.When the Mn content increases from 3.1%Mn to 9.0%Mn,the number of oxide inclusions does not increase significantly,but the average size increases,which is a main reason for the plastic decrease caused by the high 9.0%Mn content.6.For 5.8%Mn and 9.0%Mn experimental steels at the strain rate of 0.005 s^-1,the decrease of large angle grain boundary leads to the crack propagation and reduces material plasticity when the deformation temperature increases from 350?to 650?.Under the same deformation conditions,the proportion of large angle grain boundary and the number of?3 twins in 5.8%Mn experimental steel are higher than that in 9.0%Mn experimental steel,which lead to the better plasticity of 5.8%Mn experimental steel.7.The deformation structure of different Mn addition steels are mainly affected by the evolution of austenite phase dislocation structure at the deformation condition of 0.005 s^-1/350?.High density dislocations mainly occurs in austenite phase.In 5.8%Mn experimental steel,a large number of dislocation cell substructures appear on austenite phase,which are conducive to the division and decomposition of large grains and the refinement of grains;in addition,a large number of twins appear in steels.there are a lot of dislocation entanglements in austenite phase and some twins in 3.1%Mn experimental steel,and the plasticity is reduced.For9.0%high Mn experimental steel,there are a large number of dislocation cells substructure in austenite phase and deformation bands with high density dislocation areas,which is the reason that the plasticity of the 9.0%Mn steel is lower than 5.8% Mn experimental steel.