CRYSTALLIZATION, THE CAST STRUCTURE AND BUBBLE FORMATION IN HIGH NITROGEN STEELS

摘要

In the paper, the results of experimental research concerning the precipitation of nitrogen in the form of gas blowholes during the crystallization of supersaturated Fe-N, Fe-O-S-N alloys and 1Cr13 and Cr18Ni10 steels have been described. It has been found that the precipitation of nitrogen gas blowholes is more intensive and the pressure P_(N_2) is higher at low contents of surface active elements, i.e. oxygen and sulfur. At the concentration ([percent O]+0.5percent [percent S])>=300 ppm, microingots exhibited a compact macrostructure without gas blowholes. The results of a kinetic analysis of the process of desorption of nitrogen and the thermodynamics of the investigated solution (including surface tension) confirm that the surface reaction plays a decisive role in the formation of gas blowholes. For this reason, it is possible to eliminate the formation of gas blowholes in ingots of ferritic and ferritic-austenitic steels by introducing such SAE admixtures, as Sb, Te or Se. The formation of nitrogen gas blowholes was not found during the crystallization of austenitic steels. Analytical expressions have been obtained, which define the amount of nitrogen releasing into gas blowholes and describe the conditions for producing ingots or castings of a compact structure at cooling rates of approximately 10~3 K/s.