The Hot Deformation Behavior And Microstructure Evolution Of904L Superaustenitic Stainless Steel

904L superaustenitic stainless steel (SASS) is a kind of low carbon high alloy steel.Due to high additions of alloying elements, such as chromium, molybdenum, nickel andcopper, etc, this material exhibits high strength, high hardness, high toughness, goodweldability and excellent corrosion resistance. However, in actual production, highalloying degree in904L leads to high technology requirements and producing difficulty.During hot rolling production, the delamination and surface cracking can occur easily,which can seriously degrade the quality and yield rate of the products. So, the hotdeformation behavior and the microstructure evolution of904L superaustenitic stainlesssteel were studied in this paper, attempting to provide the theoretical basis forestablishing and optimizing the hot rolling process parameters of904L and improvingthe quality and yield rate of the products effectively.The dynamic recrystallization (DRX) behavior of904L SASS with the structure ofas-cast isometric crystal was studied by isothermal and constant strain rate hightemperature hot compression tests conducted in the temperature range of900-1200°Cand strain rate range of0.001-10s-1. The effects of deformation parameters(deformation temperature, strain rate and strain) on the DRX fraction andmicrostructure evolution were investigated. It shows that high temperature, low strainrate and large strain promote DRX behavior. DRX softening mechanism is alsoanalyzed by metallographic observation and transmission microscope observation andthe grain boundary bulging and subgrain induced nucleation are the DRX mechanism.The apparent activation energy of DRX is calculated to be443KJ/mol and Z values under different deformation conditions are obtained. The critical condition for theinitiation of DRX is discussed. The linear relationship between critical and peak strain isεc≈0.59εpand the linear relationship between critical and peak stress is σc≈0.98σp.Besides, in order to predict and control the process of DRX, the DRX kinetics model isestablished to be X x=1exp[0.0654[(ε εc)/εp]1.73dr]. The DRX grain size modelcan be described asDdrx=3.28×105Z0.27. This can provide a reference for actual hotrolling production.The metadynamic recrystallization (MDRX) behavior of904L SASS with thestructure of rolling palate solid solution state was studied by two-pass isothermal andconstant strain rate high temperature hot compression tests conducted in the temperaturerange of950–1150°C, strain rate range of0.05–5s-1and interval time of1–581s. Theeffects of deformation parameters (deformation temperature, strain rate, deformationdegree and interval time) on the MDRX fraction and microstructure evolution wereinvestigated. The fraction softening is calculated by0.2%offset method. It shows thatthe MDRX fraction increases with the increase of temperature, strain rate, deformationdegree and interval time. MDRX softening mechanism is also analyzed bymetallographic observation and transmission microscope observation. The MDRXmechanism is the migration of crystal boundary, the collapse of grain boundary,annihilation of dislocation in the interior of grain and then the formation of grainswithout dislocation. The apparent activation energy of MDRX is calculated to be385.85KJ/mol by linear regression. Moreover, in order to predict and control the process ofMDRX, the MDRX kinetics model is also established tobe XMDRX=1exp[0.693(t/t)0.530.5]. By comparing with predicted values andmeasured values, the correlation coefficient is obtained to be0.98and the average errorvalue is0.03, which shows that the accuracy of the model is relatively high. In the end,to control the MDRX grain size under different deformation conditions, the MDRXgrain size model is established to beDMDRX=2.16×104Z0.21.