Effect of nitride on strength and impact property of rack steel
When smelting rack steel with steel ingots,due to the absorption of N during the casting process,the N content in the steel is relatively high,which affects the solid solution B content in the steel,reduces the hardenability of the steel plate,and affects the performance of the center the steel plate.In order to improve the strength and toughness matching of the steel ingot rolling gear steel,the addition of Al or Ti elements to reaction with N in the steel ensures sufficient solid solution B in the steel plate,improves the hardenability of the steel plate,and increases the proportion of tempered sorbite in the center of the steel plate.By comparing the properties and microstructure of steel plates,it was found that both adding Al and Ti can improve the hardenability and strength of the steel plate.The steel plate with Ti has better hardenability and higher strength,but its low-temperature impact absorption ener-gy decreases.Analyzing the impact fracture surface of the steel plate,it was found that there are large particles of TiN inclusions in the steel plate.Cracks exist between the inclusions and the matrix or between the inclusions them-selves,which become the source of cracks in the impact test and reduce the toughness of the steel plate.Comparison of steel plates with Al and Ti added Statistical analysis of inclusions above 2 μm shows that there are a large number of large particle TiN inclusions in Ti steel,with a maximum inclusion size exceeding 15 μm.There are still a small a-mount of TiN inclusions in the Al added steel plate,but the number and size of TiN inclusions have decreased,and AlN has not significantly increased.In situ analysis and comparison were conducted between the Al element in the Al steel plate and the Ti element in the Ti steel plate.It was found that the Al element is negatively segregated at the center and has a lower degree of segregation.The Ti element is positively segregated at the center and has poorer uniformity,with local segregation.Thermodynamic calculations indicate that when titanium is used for nitrogen fixa-tion,TiN will precipitate at the solidification front,resulting in a higher precipitation temperature;when using alumi-num nitrogen fixation,if the mass fraction of aluminum is not less than 0.08％,AlN precipitates in the solid state at a temperature higher than BN,which can ensure the solid solution boron content in the steel plate,ensure the harden-ability of the steel plate,and control the size of TiN precipitates in the steel.
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