Analysis On Cleanliness Of Oil Casing Steel In The Production Process And Critical Process Optimization

In order to improve the cleanliness of oil casing steel produced by one steel plant,sampled during smelting,refining and casting process,analyzed steel cleanliness through metallurgical microscope,oxygen and nitrogen analyzer,scanning electron microscopy,bulk sample electrolysis method and other methods.On this basis,researches on optimizing deoxidization process and refining slag were carried out,the main conclusions reached are as follows:The cleanliness analysis of oil casing steel shows that the main problems of the production process are as follows: deoxidization process is bad for removal of alumina inclusions;the value of CaO/Al2O₃ is high,which is bad for rapid slagging and absorbing inclusions;reoxidation during pouring is serious.The primary deoxidation process of one steel plant concludes adding aluminium during tapping and refining,the new deoxidation process consolidating deoxidation during tapping.The experiment result shows that the amount of large inclusions in steel is fewer by new process,especially for steel before refining.The type of inclusions before refining evolves from metasilicate to calcium aluminate,however,the type of inclusions after refining does not change.Melting temperature of CaO-SiO₂-Al2O₃-MgO refining slag is studied by the hemispherical method,the compositions of refining slag whose flowing temperature is below 1370? are as follows: w?MgO?=6%,w?CaO?=50%⁵6%,w?SiO₂?=5%¹5%,w?Al2O₃?=23%³9%.The viscosity of refining slag is studied by rotary cylinder,it shows the viscosity is lower than 0.5pa·s at 1500?,when each composition is as follows: w?CaO?=50%⁵9%,w?SiO₂?=5%¹5%,w?Al2O₃?=23%³9%,w?MgO?=6%⁸%.Combined with laboratory research results,the ideal slag composition is put forward:w?CaO?=50%⁵6%,w?SiO₂?=10%¹5%,w?Al2O₃?=23%³0%,w?MgO?=6%⁸%.The field test shows flowing temperature of the optimized slag is lower than 1370?,sulphur content is less than 0.005%,the desulfurization effect improves obviously,but the deoxidization effect does not improve.After optimization of deoxidation process,refining slag and protective casting,the industrial experiment was carried out.The result shows that: the total oxygen content in steelis less than 20 ppm after refining,nitrogen content of steel in tundish is less than 51 ppm,reoxidation degree reduces than before.No strip shaped Al2O₃ and spherical MnO-SiO₂-Al2O₃ inclusions were found,the number of large inclusions in slab is only 0.4×10-3 per square millimetre,nonmetallic rating completely meets the requirement of product standards.