| The Crl5Mn9Cu2NilN austenitic stainless steel belongs to 200 series austenitic stainless steel,which is a type of stainless steel with nitrogen and manganese partly replacing nickel to obtain the austenitic microstruture.Its excellent mechanical property and low cost make it to be widely used in various fields.The recrystallization microstructures of austenitic stainless steel are obtained through controlling hot working process to get excellent property of material,which is especially important. On the industrial condition,hot rolling is vital to producing the Crl5Mn9Cu2NilN austenitic stainless steel strips.Because chemical components of this steel are more different than these of conventional Cr-Ni austenitic stainless steel,and its hot workability shows great differenence,the hot working characteristic of this steel has not been clearly recognized.Therefore,the working process of hot rolling has not been perfected.During hot rolling,some products prone to defects,such as edge cracks and surface cracks,which seriously affect quality of products.These problems are usually solved by method of hot processing maps,so studying high temperature deformation characteristic of the Crl5Mn9Cu2NilN austenitic stainless steel has very important theoretical significance and application value.In this paper,the constant temperature and constant strain rate compression test method was performed by using the Thermorestor-W thermal/mechanical testing machine.In the deformation temperature range from 950℃to 1200℃and strain rate range from 0.01s-1 to 2.5s-1,the hot deformation behavior and hot workability were investigated by true stress-true strain curves and hot deformation microstructures of Crl5Mn9Cu2NilN austenitic stainless steel of two different nitrogen content.On the basis of the principles of dynamic materials model,the processing maps of the steel were constructed at different true strains.The processing maps were synthetically analyzed connecting high temperature deformation microstructures,thus their micro-mechanisms were clearly revealed under different deformation conditions during hot working,and the optimum hot working process parameters were got,which provided a theoretical foundation for the general principles of process setting and choosing of feasible process parameters.The main conclusions are drawn as follows:1.The flow stresses of Crl5Mn9Cu2NilN austenitic stainless steel are strongly influenced by deformation temperature and strain rate during high temperature compression.It decreases with the increase of deformation temperature and the decrease of strain rate.Nitrogen has remarkable function of solution strengthening as interstitial solid solution atom.When the deformation temperature is less than 1150℃, the increasing of nitrogen content result in increasing ofthe flow stress,but the effect is not remarkable above 1150℃.2.The hot deformation activation energy of Crl5Mn9Cu2NilN austenitic stainless stecl is calculated by regression analysis method,and they are 490.79kJ/mol and 464.22kJ/mol respectively.Hot deformation activation energy of this steel is higher than conventional Cr-Ni stainless steel for high manganese content.Hot deformation activation energy is also affect by nitrogen,which is decreased 26.27kJ/mol due to increasing 321ppm nitrogen.Their material constants are respectively calculated by true stress-true strain curves,and finally the hot deformation constitutive equations are got.3.At constant strain rate,with the increasing of deformation temperature,the microstructure evolution rule of Crl5Mn9Cu2NilN austenitic stainless steel is from dynamic recovery to partial dynamic recrystallization,finally fully dynamic recrystallization.The recrystallization grain size is rarely affected by strain rate when nitrogen content is lower,but its microstructure morphology is obviously affected,viz. austenitic grain boundary is curve state at strain rate of 0.01s-1,and is straight line state at strain rate of 2.5s-1.When nitrogen content is higher,the recrystallization degree is gradually increasing with the increase of deformation temperature and decrease of strain rate.At constant strain rate,the full dynamic recrystallization microstructure can be got above 1150℃.But at constant deformation temperature,the recrystallization grain size decrease with the increase of strain rate.4.The hot processing maps of Crl5Mn9Cu2NilN austenitic stainless steel with two different nitrogen content were analyzed,the result shown that "safe" hot working window was expanded by increasing nitrogen.When nitrogen content is lower,the efficiency of power dissipation hardly varies with increasing of true strain,and its distribution and change tendency are similar,but the flow instability domain gradually decrease with increasing of true strain.The preferred hot working process parameters are deformation temperature ranging from 1170℃to 1200℃and strain rate from 0.1s-1 to 2.5s-1,which is a domain of fully dynamic recrystallization.Nevertheless,when nitrogen content is higher,the flow instability domain moves from high temperature to low temperature with increasing of true strain,this indicates that "safe" hot working window is expanded with increasing of true strain.The appropriate hot working process parameters are shown:the first domin is in the deformation temperature range from 1100℃to 1200℃and strain rate range from 0.01s-1 to 2.5s-1,and the second domain is in the deformation temperature range from 1020℃to 1100℃and strain rate range from 0.4s-1 to 2.5s-1.The optimum process parameter is the deformation temperature range from 1120℃to 1200℃and strain rate range from 0.18s-1 to 2.5s-1, which is a domain of fully dynamic recrystallization and the efficiency of power dissipation is more than 30%. |
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