Study On Hot Forging Cracking And Grain Size Refining Of Heavy High Nitrogen Steel Retaining Ring

For heavy retaining rings,the deformation processes are long and complex.The local strain rate of forging surface is very different.However,the cracking mechanism and cracking process of high nitrogen steels under different strain rates are not clear.If expanded rings are subjected to further annealing,the grain structure may be refined by static recrystallization.However,the dynamic recrystallization after cold deformation has not been reported in detail.In this study,the problems of cracking,coarse grain and mixed grains in the process of forging of high nitrogen steel ring were discussed.The mechanism of hot forging cracking under different strain rates was revealed,and the evolution of the microstructure after cold rolling and rapid high temperature annealing were analyzed.The following is the main conclusion:(1)The crack nucleation sites are primarily located at GBs and are unaffected by the strain rate at various deformation temperatures.At high deformation temperature 1200?,a higher strain rate is beneficial for grain refinement and to prevent hot cracking;However,at very low strain rates(0.001 s-1),DRV occurs and crack propagation is transgranular and along low-angle GBs.(2)At moderate deformation temperature(1100?)and medium strain rate,the high dynamic recrystallization ratio and the small dynamic recrystallization grain make the hot ductility become good.When the temperature is low,the strain gradient from grain to GB increases with the decrease of strain rate.GB sliding accompanies grain rotation and does not reduce the hot ductility.(3)At the triple junctions,the dislocation translates into low-angle GBs.With the increase of isothermal time,the low-angle GBs grows into the high GBs and recrystallization nuclei are formed.At the front of recrystallization,the low-angle GB is formed by dynamic recovery,and then it turns into a high angle GB;If a prior HAGB bulges,a LAGB will appear to aid with nucleation between the two deformed grains.If the dislocation density is too low to form a LAGB in one grain,a new HAGB generates in another grain.(4)The static recrystallization of 18Mn18Cr0.6N steel after cold tension can completely eliminate the cold deformation texture.After two cycles of 40% cold tension and annealing at 1000 °C,the grain size number changed from 1.5 to 8.5;the resulting increments in yield strength,ultimate strength,the linear strain hardening rate,and the hardness were 9.2%,16.3%,19.9%,and 7.2%,respectively.(5)Appropriate cold deformation and annealing treatment could be obtained fine-grained heavy retaining rings.