Research On Microstructure Evolution Of ESR Mn18Cr18N Austenitic Stainless Steel During Hot Deformation

Mn18Cr18N,a high-nitrogen austenitic stainless steel,is widely used to manufacture retaining rings of generators due to its distinctive advantages such as high strength and toughness,non-magnetic property and excellent stress corrosion resistance.And the casting structure of traditional grains are coarse and have serious defects due to the restriction of the metallurgy.However,the billet quality has been improved greatly because of the process of ESR.Although the defects are greatly reduced,the grain structure basically consists of coarse columnar crystals.Mn18Cr18 N steel as a kind of non-phase steel,the organizational performance can't been optimized by the heat treatment process.Therefore,the technology of grain refinement becomes the key to improve the organization's performance.As we know,most of retaining ring usually made by the forged organization.And the manufacturing process is various and complicated in the process of forging.So,it is very important to study the hot deformation behavior of the as-cast microstructure of electroslag remelting Mn18Cr18 N steel in the process of forging process,which is of great practical significance.The contents of this paper are as follows:First of all,uniaxial hot compression test of the ESR Mn18Cr18 N austenitic stainless steel was conducted,the stress-strain curves under different deformation conditions were obtained and the deformation law of this steel was analyzed.The results show that the steel is sensitive to the strain rate and the strain rate sensitivity is obvious.Secondly,Research on the regularity of dynamic recrystallization of as-cast Mn18Cr18 N austenitic stainless steel microstructure under different deformation conditions were conducted.Through to study the effect of temperatures,strain and strain rate on dynamic recrystallization softening behavior,and the constitutive equation and the dynamic model of the steel were established.With the help of optical microscope(OM),the dynamic recrystallization organization fraction is observed.By using data-fitting of origin software,the recrystallization fraction model and grain size model were constructed.The results show that the dynamic recrystallization grain is mainly produced from the original grain boundaries with "necklace" shaped pattern.With the rising temperature and reduced strain rate,the dynamic recrystallization fraction increases gradually.Compared to the forged organization,a result was found that as cast microstructure deformation activation energy was higher and the dynamic recrystallization activation energy was lower,which meant that the microstructure of plastic deformation was difficult to occur,but prone to dynamically recrystallize.Then,analysis the dynamic recrystallization energy dissipation mechanism of micro-deformation microstructure was carried on,then the hot processing map was established under different strain,which was based on dynamic material model.A hot processing map helps to analyze the thermal effect of process parameters of the microstructure,providing a theoretical basis for the optimization of supporting ring processing technology.Finally,double passes for hot compression tests of different temperatures,holding time and strain were conducted by the Gleeble-1500 D thermal simulation machine in view of the Mn18Cr18 N austenitic stainless steel.The results show that the higher the temperature is or the longer the interval holding time is,the greater the percentage of static recrystallization will be.