Effect Of Pre-deformed Microstructure Of Fe-15Mn-22Cr-0.9N Austenitic Stainless Steel On Nitride Precipitation And Mechanical Properties

High-nitrogen austenitic stainless steels have many advantages,such as excellent ductility,anti-corrosion and nonmagnetic,which is widely used in the petrochemical,medical and other industrial applications.However,due to the supersaturated nitrogen content,it is susceptible to massive nitride precipitation during thermomechanical treatment,which causes serious brittleness problems.Therefore,the property enchancement of high-nitrogen austenitic stainless steel requires the study of the mechanism of nitride precipitation,in order to provide a theoretical basis for the designing and production high-nitrogen steels with outstanding strength-plasticity combination.The current studies on nitride precipitation mainly focus on the influence of isothermal aging on the precipitation in the solid solution state.But the influence of deformation microstructure on nitride precipitation mechanism is less explored.In this paper,the solid solution Fe-15Mn-22Cr-0.9N austenitic stainless steel is rolled at room temperature and 600℃,respectively,to obtain cold-rolled and warm-rolled sheets with different rolling amounts,and then the rolled structure is subjected to aging treatment.The mechanical properties of the samples are tested using a multifunctional mechanical testing machine with a combination of experimental means such as electron scanning micro analysis,electron backscattering technique（EBSD）and transmission electron micro analysis to systematically investigate the effect of rolling temperature on the deformed tissue and the effect of deformation structural characteristics on nitride evolution,to explore the technical path for coupling deformation with phase transformation to prepare high strength-high plasticity high nitrogen austenite stainless steel.The main findings of this paper are as follows:（1）The yield strength of solid solution high-nitrogen steel is 440 MPa,the tensile strength is 1430 MPa and the uniform elongation is 0.42,and it has high thermal stability with no phase transformation occurs after aging at 600℃for 5 h.The yield strength of the experimental steel can be significantly improved by rolling deformation.The tensile strengths of samples with cold rolling amounts of 20%,40%,60%and 75%are 1252 MPa,1579 MPa,1803 MPa and1875 MPa,respectively;samples with warm rolling amounts of 28%,40%,60%and 70%are The tensile strengths are 1393 MPa,1462 MPa,1565 MPa,and 1670 MPa,respectively.（2）It is found that temperature has an important effect on the deformation structure.Comparing the results of EBSD analysis of cold-rolled and warm-rolled samples,it is found that the number of twin boundaries of cold-rolled samples is much higher than that of warm-rolled samples,and at the same time,they have higher dislocation density.Transmission electron microscopy（TEM）analysis results show that there are a large number of deformed nano twins in the former,and there are a large number of dislocations in the twin lamella;In the latter,there are fewer twins and larger twin boundary spacing,and a large number of dislocation cell structures are formed.（3）The deformation microstructure has an significant influence on the nitride precipitation in subsequent annealing.Scanning electron microscope（SEM）analysis shows that the nitride precipitation of cold-rolled samples after annealing is much greater than that of warm rolled microstructure,and mainly precipitated in twin lamella;In the warm rolling structure,the cellular structure precipitated from grain boundary is the main one.TEM results show that the precipitation is rich in chromium and M₂₃C₆ with face centered cubic structure,in which C atom was partially replaced by N atom to form M₂₃（C,N）₆ compound.（4）Brittle fracture of cold rolled-annealed samples is occurring in tensile test.After annealing treatment,the fracture strain and the fracture stress of cold-rolled decreases sharply.The brittle fracture is caused by the precipitation of M₂₃（C,N）₆ at the grain boundary and twin boundary.Due to a large amount of precipitation in the sample,the micro crack is easy to nucleate and expand in M₂₃（C,N）₆,resulting in brittle fracture of the material.（5）The tensile plasticity of the warm rolled-annealed samples is much higher than that of the cold rolled-annealed samples.The uniform elongation increases from the initial 0.03 to0.13-0.16,and the fracture strain and fracture stress are reduced.The fracture mode has the characteristics of delamination,that is,the crack propagates along the tensile direction in grain boundary to form a significant delamination,and the fracture has the characteristics of ductile rupture both macroscopically and microscopically.Therefore,warm rolling and annealing is an effective process path to obtain high strength,high plasticity and high nitrogen austenitic stainless steel.