Study On Surface Crack Formation Mechanism And Control Of S32101 Duplex Stainless Steel In Hot Rolling

With the development of science and technology,traditional metal materials with single excellent properties can no longer be used in complex environments.The development of new metal materials with multiple excellent properties has became a new research focus in the iron and steel industry.Among them,the research progress of duplexed stainless steel as a kind of new stainless steel is particularly outstanding.At present,the main industrial production of duplex stainless steel products is austenitic-ferritic duplex stainless steel.Austenitic-ferrite duplex stainless steel has high strength,good plastic toughness,excellent corrosion resistance and weldability,which are determined by ferrite and austenite phases.In order to maintain the two-phase ratio of ferrite and austenite,which needs a certain amount of Ni element to be added,which leads to the increase of production cost.In order to reduce the production cost,The Same austenitized stable element-N element is used by Finland's Otto Kumple company to replace Ni element and develop economic duplex stainless steel.However,in the process of high temperature hot rolling production,the surface quality of the finished products is poor because of the poor thermoplasticity of economical duplex stainless steel.In particular,there are a lot of cracks on the surface of the plate,which increases the subsequent labor cost.The object of this study is S32101 duplex stainless steel.In order to explore the correct production process,OM,SEM,XRD,TEM,EBSD,Thermecmastor-Z thermal simulation machine,box resistance furnace and Vickers microhardness tester were used to study the phase transition law of S32101,the precipitation behavior of hard precipitate and the deformation characteristics during high-temperature processing.Firstly,the solid solution test was carried out,and the hard precipitate of S32101 duplex stainless steel was determined as Cr₂N by SEM observation and TEM diffraction pattern calibration.At the same time,the phase transition rule and precipitation behavior of S32101 duplex stainless steel were determined with XRD test,and then the suitable processing temperature range of S32101 duplex stainless steel was obtained.After the proper processing temperature range was determined,and thermecmastor-Z thermal simulator was used to simulate different deformation rates and stages,and the thermal processing diagram of S32101 duplex stainless steel with the strain condition of 0.7 was constructed.The deformation characteristics of S32101duplex stainless steel were studied.It is found that within the appropriate processing temperature range,the instability region still exists and the instability region is concentrated in the low deformation rate region.In view of the above phenomenon,the microstructure of samples under different deformation conditions was observed in this study,and it was found that the instability phenomenon was caused by microcracks at the phase boundary or in the ferrite matrix.In the initial stage of the study of deformation characteristics,it was determined that the microcracks were induced by the stress concentration caused by hard precipitates and the uneven distribution of two corresponding variations.However,it was found that there were no microcracks on the surface of the samples with high deformation rate in the presence of hard precipitates,which was contrary to the initial conclusion.Finally,in this study,EBSD was used to detect the strain distribution under different deformation conditions,and the dislocation migration in the deformation process was observed by TEM,thus the deformation characteristics of S32101 duplex stainless steel were characterized.It is judged that the deformation of S32101 duplex stainless steel first occurs in the soft ferrite and then diffused to the austenite.When the deformation rate or amount is large,the microcrack initiation induced by hard precipitates can be inhibited and the surface quality of S32101 duplex stainless steel can be improved.