Hot Deformation Characteristics Of Lean Austenitic Heat-resistant Steel

In recent years our country pays high attention to the development of circulatingfluidized bed boiler technology with high efficiency and low pollution and makeshigher requirements to the performance of related components. Austenitic heatresistant stainless steel has been widely applied in the circulating fluidized bed boilersuch as combustion chamber and the separator because of its good strong heatresistance and oxidation resistance. On the other hand, austenitic heat resistantstainless steel has high alloy content, while the chromium, nickel resources in Chinaare relatively scarce, hence our country in urgent need to develop aconservation-saving austenitic heat resistant steel varieties with high added value atpresent, to relieve the the contradiction of components between high performance andhigh resource consumption.A certain type of lean austenitic heat resistant stainless steel as the main researchobject in this paper has good elevated temperature property which is better than310(25Cr-20Ni) and close to800H (23Cr-35Ni), the material obtains its heat resistantproperties by precise control of micro alloy additions, especially the rare earths (suchas Ce, Y) combined with nitrogen is on the nominal composition basis of21Cr-11Ni.So far, many researches of the21Cr-11Ni-N-RE steel which focused on the reliabilityassessment for the service properties, such as creep, oxidation, hot corrosion, etc.However, there is still a lack of knowledge concerning the microstructural evolutionand mechanical behavior in hot working process, and also, the alloy content is higherand security hot working window is narrow, so it is very necessary to study the hotdeformation behavior and explore the appropriate hot working window of this leanaustenitic heat-resistant stainless steel.Based on the the optimization of the lean austenitic heat-resistant stainless steelmicrostructure, this article investigated the thermal deformation constitutivecharacteristics, established processing maps and analyzed the rule of microstructureevolution, at the same time, discussed the response rule of dynamic recrystallizationbehavior under different thermal deformation process parameters based on the numerical simulation technology, by using thermal simulator system within thetemperature of900-1200℃and the strain rate of0.01-10s-1, the main results are asfollows:(1) Hot deformation behavior of test steel is investigated by the use of hotcompression simulation experiment. Relationships between deformation temperature,strain rate and flow stress are analyzed: the flow stress value decreases withincreasing temperature and decreasing strain rate. Material constants are strongdependent on the strain and the values of α, n, Q, lnA vary in the range of0.006-0.0095MPa-1,4.84-6.24,447.33-523.07kJ/mol,38.11-43.58, respectively. Thepeak stress constitutive model and a constitutive equation considering the effect ofstrain have been developed. Flow stress values predicted by the constitutive modeldemonstrate a well agreement with the experimental results, so the constitutive modelis correct.(2) Based on the quantitative analysis carried out on the hot compressionexperiment results, the critical strain model describing the hot deformation processand the dynamic recrystallization kinetics model predicting the organizationalevolution are established.(3) Based on dynamic material model and Prasad instability criterion, the powerdissipation efficiency and instability parameter are determined under different hotdeformation conditions, and then processing maps are established and analyzed.Under the large deformation condition (ε=1.0), the optimum hot working domain is at1150-1200℃and1-10s-1, meanwhile the material has the peak power dissipationefficiency (38%) and the full dynamic recrystallization microstructure with uniformand fine.(4) Based on the microstructure analysis under different hot deformationconditions, it is found that deformation temperature and strain rate have an importantinfluence on the dynamic recrystallization microstructure evolution. Dynamicrecrystallization volume fraction and recrystallization grain size increase withincreasing temperature and decreasing strain rate.(5) Recrystallization microstructure evolution process is simulated using the established constitutive model with the dynamic recrystallization kinetics model,results show that the numerical simulation results are in good agreement with physicalsimulation results, and the recrystallization kinetics model is proved to be correct.