Hot Deformation Behavior And Processing Map Of Austenitic Stainless Steel Containing Niobium

The steam generator heat-transfer pipes are the core of the key components of Ultra-Supercritical?USC?,and the high parameters and high reliability of equipment is determined by heat-transfer pipes,therefore it demands high service performance of materials.The key heat pipe of ultra supercritical unit in China is mainly dependent on imports,lacking the new materials with independent intellectual property rights and the complete sets of manufacturing molding technology now.It is of importantance for coal-fired power generation technology to develop new materials for heat-transfer pipes in our country.Based on the above application background,this subject has developed two kinds of the new austenitic stainless steel that are based on Cr-Ni austenitic stainless steel by adding different niobium contents and other alloying elements.Niobium can not only enhance the corrosion resistance of the material,but also can improve the room temperature and high temperature mechanical properties of austenitic stainless steel.Ultra-supercritical thermal power boiler pipes need thermal piercing and other processing technologies before being equipped.Therefore,the best hot processing system can be determined by studying the hot deformation behavior so as to control the evolution of the microstructure and the final performance of the product.A single pass hot compression test of the new austenitic stainless steel containing niobium was carried out in the deformation temperature range of 950¹100?and strain rate range of 0.01¹s^-11 by using the thermal simulation test machine.The rheological stress curve and deformation microstructure of tested steel under high temperature was observed and analyzed.The modified constitutive model coupled with strain was established.The critical strain of dynamic recrystallization was predicted,and the hot working maps under different strain were constructed.The conclusions are as follows:1?The compression data under high temperature was obtained and the rheological stress curves of tested steel were analyzed by using the Origin software,the effects of different hot deformation parameters on the rheological stress and microstructure evolution of the new austenitic stainless steel containing niobium were revealed systematically with metallographic microscope?OM?and scanning electron microscope?SEM?.The results showed that the rheological stress of the tested steel is sensitive to the strain rate and deformation temperature.when the strain rate decreases and the deformation temperature increases,the rheological stress decreases and the degree of dynamic recrystallization is greater.2?The hot deformation activation energy of the new austenitic stainless steel containing niobium was obtained by regression analysis method.The hot activation energy of the G1 and G2 tested steel was 362.748 kJ/mol and 365.111 kJ/mol,respectively.At the same time,the modified constitutive model of coupled strain was established and the rheological stress of the new alloy was predicted.The correlation coefficient and the average relative error between the predicted values of G1 tested steel and its compression values are 0.99785 and 2.5%,respectively.The correlation coefficient and the average relative error between the predicted values of G2 tested steel and its compression values are 0.9868 and 3.3%,respectively.The critical condition of dynamic recrystallization of tested steel was confirmed by using three polynomials fit the relationship curve of ln?-?,the mean ratio of critical strain??_c?to peak strain??_p?of G1 and G2 tested steel under different deformation conditions is0.6858 and 0.6994,respectively.3?Based on the dynamic material model?DMM?,the hot processing maps of the austenitic stainless steel containing niobium were constructed.The safe zone and the rheological instability zone of the tested steel were determined under different hot deformation parameters by the hot processing maps so that a reasonable hot processing system was worked out.According to the hot processing maps and microstructure,the best hot processing parameters of G1:1050¹100?and0.0316⁰.1s^-1;the best hot working parameter of G2:1000¹050?and 0.039⁰.1s^-1.