Research On Microstructure Control And Strengthening Mechanism Of High Nitrogen Austenitic Stainless Steel

This subject stems from the Collaborative Innovation Center of Heavy Machinery of Yanshan University and the scientific and technological problems that need to be solved urgently in an enterprise's production.The research and development of nickel-rich high-strength austenitic stainless steels is funded by the Hebei Provincial Science and Technology Plan Project.Relying on this,the purpose is to study the microstructure changes and strengthening mechanism of high-nitrogen stainless steel under hot rolling,heat treatment,cold rolling and other processes.In this paper,four high-nitrogen austenitic stainless steels with different compositions were selected as the research object.The influences of controlled rolling parameters?finishing temperature and cooling method?on the microstructure and mechanical properties of the steel were studied.Precipitation law in solid solution and aging process and cold work hardening behavior of test steel were also systematically studied.At the same time,the microstructure deformation characteristics and the strengthening mechanism were clarified in plastic deformation processes.The main results are as follows:?1?Through the hot rolling treatment of 25-N test steel,the effect of the final rolling temperature and cooling method on the microstructure was studied,and the strengthening mechanism under different hot rolling processes was explored.The experimental results show that,with the decrease of the final rolling temperature,the austenite grains change from the partial equiaxed to the flattened elongate shape,the content of ferrite and the amount of precipitated spheroidal carbides in the test steel increase;tensile strength and yield ratio increase with decreasing finish rolling temperature,the elongation and impact toughness decrease,but the change in the area shrinkage ratio is not large,tensile strength of test steel in the air cooling is greater than water cooling,and the cooling method has little effect on elongation,reduction of area and the yield ratio.?2?Through the heat treatment of the 25-N test steel after hot rolling,the effect of solid solution and aging on the microstructure and mechanical properties of the hot-rolled test steel was studied.The test results show that the precipitated carbon and nitride in the test steel increase with time,the most sensitive precipitation temperature is 750?,the main precipitate is Cr₂₃C6 and Cr₂N and the precipitation process is:precipitation along the intersection of the three grain boundaries?cell??precipitation along the grain boundary?bar?and a small amount of precipitation along the grain inside?intragranular growth?pearlitic lamellar tissue.In addition,the strain hardening mechanism of the test steel after heat treatment mainly includes dislocation strengthening,twin strengthening,and stacking fault strengthening.At the beginning of deformation,dislocation strengthening plays a dominant role;when it is??28?.005later,twin strengthening and fault strengthening begin to play a main role.?3?Through cold deformation rolling of 19-N test steel and 13-N test steel,the microstructure and properties of different carbon content test steels under different cold deformation conditions were studied.The metallographic organization shows that as the deformation increases,the microstructure of the test steel changes as follows:twins and slip?single-slip dominated?coexist?bending deformation of the grain boundary,impeding the movement of the slip line?slip line passing through crystals,and obstruction weakened?The twins are cut by slip lines and the twin boundaries tend to be blurred.The tensile test results show that the cold deformation can significantly increase the strength of the test steel,but the plasticity becomes worse;at low strain?10%?,the19-N test steel with low-carbon has a slightly greater strain hardening exponent,when the deformation is greater than 10%,the 13-N test steel with high-carbon showed stronger work hardening ability.