Study On Microstructure And Properties Of Mn-N Saving Type High Strength Duplex Stainless Steel

Lean duplex stainless steel is one of the most promising materials among stainless steel materials,and has been widely used in fields such as chemical,petrochemical,and seawater desalination.Lean duplex stainless steel made of austenite(γ)and ferrite(α)Composed of a two-phase structure,it has high corrosion resistance and excellent mechanical properties.However,in recent years,the engineering field has put forward higher requirements for the performance of lean duplex stainless steel.Both solid solution treatment and cold rolling process can improve their mechanical properties by regulating the microstructure.Therefore,in order to better improve the structure and properties of lean duplex stainless steel and expand the practical application range of this type of material in different engineering fields,it is necessary to develop new high-performance lean duplex stainless steel.This article focuses on the self-developed and designed Fe-0.02C-20Cr-6Mn-1Ni-0.2N-0.35Si lean duplex stainless steel.The effects of solid solution treatment on the microstructure,mechanical properties,and corrosion resistance of this new type of lean duplex stainless steel were studied using light microscopy,scanning electron microscopy,electron backscatter diffraction technology,X-ray diffraction,tensile experiments,and electrochemical workstations.At the same time,the internal relationship between the cold rolling deformation process and its microstructure and mechanical properties was also explored.The main results are as follows:(1)As the solid solution temperature increases,austenite gradually transforms into ferrite,and the two-phase structure changes from a strip like structure to an island like structure.Owing to the redistribution of elements,the stacking fault energy of austenite also increases from 18 m J/m~2 to 38 m J/m~2 with the increase of solution temperature,which inhibits the martensite transformation,leading to the weakening of TRIP effect.As the solid solution temperature increases,the pitting potential of the experimental steel first increases and then weakens.At 1100℃and 1150℃,the pitting potential is almost the same,indicating that the passivation film on the surface of the experimental steel has higher stability and better inhibition effect on pitting corrosion at the two temperatures.As the proportion of ferrite phase increases,the concentration of Cr in the ferrite decreases,leading to an increase in corrosion rate and ferrite becoming the preferred phase for corrosion.When the solid solution temperature is 1100℃,the tensile strength can reach 901.1 MPa,the yield strength is 561.5 MPa,and the elongation can reach 69.8%.The pitting potential reaches347.3 m V,which combines mechanical properties and good corrosion resistance,demonstrating superior comprehensive performance.(2)During cold rolling with low deformation,the strain is mainly distributed on austenite,and strain induced martensite(α′)is formed.The larger the cold rolling deformation,the more strain induced martensite content.When the deformation reaches70%,the content of martensite can reach 42.2%.Through electron backscatter diffraction technology,it was found that there is aγ→εmartensite(transition phase)→α′martensite andγ→α′.There are two kinds of martensite transformation mechanisms of martensite.With the increase of cold rolling deformation,the strength of the cold rolled specimen significantly increases,while the elongation sharply decreases.When the deformation amount increased to 70%,the yield strength and tensile strength of the cold rolled sample reached 1551.9 MPa and 1834.1 MPa,which were 193%and 87.8%higher than the solid solution sample,respectively.The microhardness was 538.2 HV,which was 81.1%higher than that of the solid solution sample.