The Study On The Problems Of Dephosphorization In Stainless Steel Production

This report was prepared as an account of work sponsored by Bao-Steel,The research on the problems of dephosphorization in stainless steel production was done. This research was based on the achievements obtained by Ferro-metallurgy subject Shanghai University and author in dephosphorization of stainless steelTo make up the shortage of chromium resource,the hot metal containing high chromium(25%) is to be produce by "smelting-reduction shaft".Following fur subjects were studied by the author to make sure that the hot metal containing high chromium(25%) could be use in stainless steel:1) Hot experiments and pilot experiments on desiliconization of hot metal containing high chromium(25%);2) Hot experiments and pilot experiments on oxidizing dephosphorzation of hot metal containing high chromium(25%),and treatment of used slag containing Ba⁺⁺ which is harmful to health;3) Hot experiments and pilot experiments on reducing dephosphorzation of hot metal containing high chromium(25%),and design of reducing dephosphorization reactor,production procedure,scheme of treatment of used slag;4) Optimization of dephosphorization agent used in pretreatment of hot metal.The concept of critical desiliconization value was put forward,and the how much silicon content requirement was calculated by the author.The influences of hot metal contents on desiliconization were discussed.The w[Si]in hot metal([Cr]=22～25%) can be reduced from 0.70～0.93%to less than 0.084%.disiliconization degree could be kept higher than 80%,and the loss of[C]and[Cr]was not so high."The selective oxidation among[Cr],[P],and[C]three elements" was discussed and calculated in thermodynamics.It was presented by author that carbon in hot metal containing chromium could keep chromium efficiently during dephosphorization."The selective oxidation among[Cr],[P],and[C]three elements" was discussed and calculated in thermodynamics.It was presented by author that carbon in hot metal containing chromium could keep chromium efficiently during dephosphorization. CaO-BaO(55%)-CaF₂-Cr₂O₃/Fe₂O₃ fluxes was used in dephosphorization of hot metal containing chromium w[P]decreased by 50.77～78.13%with 100kg/t fluxes injected by air into hot metal melt.A method was invented by author to treat waste slag containing BaO,which could dissolve in water and harmful to human health.The waste slag treated with the method was analyzed no toxicity.But there are still amount of BaO in slag after treatment.So the cost and environmental problem is still a big problem that is hard to be solvedA reducing dephosphorization scheme was presented by author,which could figure out the difficulties faced by many reducing dephosphorization experiments done in 80's last century.In this scheme,the reactor was sealed and kept in strong reducing atmosphere,and dephosphorization fluxes were wire-feeded.In this experiment the original phosphorous content was 0.040～0.057%.If the fed amount of Ca reached 23 kg/t,the dephosphorization ratio could be 55.4～78.0%.The calcium utilizing efficiency was found higher in pilot test than in lab.The average the calcium utilizing efficiency in SiCa wire reached 0.203×10³(kg/%·t).The characteristics of high chromium molten iron oxidizing dephosphorizafion and reduction fluxes dephosphorization as well as the effect of carbon are discussed and analyzed.The conclusion comes from the discussion.If the mother solution of stainless steels produced by a smelting reduction process contains too high phosphorous,then oxidizing dephosphorization is suggested.Reduction dephosphorization is more suitable than oxidation dephosphorization for the treatment of stainless steel scraps containing high phosphorous and the treatment when phosphorous content is out of the steel specification during the melting of stainless steel with recycled scrap.The flux composites were optimized through experiments on fast slag-forming and equilibrium between slag and hot metal.The optimized flux was used for ladle dephosphorization of hot metal.Industry trial test showed that the amount of flux consumed decreased from 39.34kg/t to 34kg/t.