Research On Refinement Mechanism Of Coarse Grains For 316LN Steel During Inhomgeneous Hot Deformation

The main pipe is one of the most important component of nuclear equipments.It is generally on service in extremely working conditions with high temperature,high pressure,high speed and radioactive fluid.Therefore,materials with high corrosion resistance and strength are necessary for the manufacture of the main pipe,and the hot forging process is frequently used to fabricate the main pipe.During the hot forging process,however,the coarse grain microstructure with extremely large grain size inevitably appears in the material,which leads to the inhomogeneous plastic deformation and deteriorates the mechanical properties of the main pipe in turns.It is crucial to obtain the homogeneous microstructure with uniform grain size for the main pipe by improving the hot forming process.With the aim to design reasonable hot forging processes,it is necessary to study the plastic deformation mechanism of metallic materials with the coarse grain microstructure.In this work,the coarse grain of 316LN steel was considered.Based on the crystal plasticity theory,the influence of grain size and orientation on the deformation process was simulated.The results showed that there was an obvious difference between the strain of coarse grains and the strain of fine grains.The mis-orientation of grains led to the macro inhomogeneous plastic deformation.With the increase of deformation,the local slip systems of the coarse grains were activated,and partial regions of the coarse grains were rotated,resulting in the distribution of local strain.The coarse grains began to broken partially.It was also found that the compression direction perpendicular to the long axis of coarse grains was in favour of starting the potential slip systems of the coarse grains and then broke the coarse grain.Furthermore,the compression processes of billets with a coarse grain were modeled by using the commercial FEM software ABAQUS/Standard.The results showed that the difference of the equivalent plastic strain between the coarse grain and the fine grains in the large deformation region,reached the maximum value when the total reduction ratio was 50%,and reduced quickly when the total reduction ratio was larger than 50%.Therefore,the total reduction ratio should not be less than 50%in the actual manufacture processes.Based on the Gleeble-3500 hot compression tests,the mechanism of microstructure evolution and refinement of coarse grains was studied.The results showed that the refinement mechanism of coarse grains was the synthesis results of many different mechanisms,e.g.,the mechanical fragmentation and the recrystallization nucleation in the grain interior.The coarse grain was completely broken under this deformation condition of temperature 1200?,strain rate 0.01s^-1,compression ratio 50%.Based on the numerical simulations and the Gleeble-3500 hot compression tests,hot upsetting experiments were conducted to analyze the microstructure evolution of coarse grains.The results showed that the refining temperature of coarse grains should be higher than 1150?,and the total reduction ratio should be about 50%in the actual manufacture processes.