| There were series of difficulties include smelting effect,cleanness control in whole process,quality and performance stability and so on when producing medium-alloy steels based on BOF-RH-CC process.Novel structure of oxygen lance and top-bottom blowing technology,deep dephosphorization with double slag,degassing and desulfurization in RH refining process,inclusion modification with different core wires injection were studied with methods including theoretical analysis,model simulation,industrial scale experiments and comprehensive inspection and detection technology based on metallurgical process with core equipment of 80t convert and other relative facilities were studied in this thesis,with tundish auto-start technology,high level and high quality continuous casting process was realized,and the relationship between inclusions and toughness of final plate after rolling and heat treatment was discussed.the main content and conclusion are listed as followings.?1?Smelting process in BOF.Optimized nozzle structure of the oxygen lance with enlarging the nozzle angle from 12°to 13°and adjusted the arrangement of bottom blowing relatively,this could improve the general smelting efficiency and dephosphorization result,and with the new top-bottom technology,carried out the deep dephosphorization with double slag operation,got the relational expression between[C]and[P]at the end of first smelting with regression,that is[P]=0.00267×[C]2.0172.Final[P],[S]in 10CrNi3MoV in convert can be controlled below 0.0072%and 0.0050%with tapping temperature from 1650?to 1680?.?2?Reducing the oxygen consumption with higher steel temperature arriving RH,improving the providing efficiency of oxygen and temperature rising speed,giving better condition for desulfurizing and degassing.The process was optimized with inner diameter of snorkels being expanded and driving gas flow increased.Desulfurizing agent with CaO+CaF2 was added with 10%MgO in RH refining procedure,this improved the capacity coefficient Ka of dehydrogenation from 0.0048s-1to 0.0056 s-1,Nitrogen removal rate reach over 15 percent,[N]in steel was controlled below 35ppm,desulfurization rate between 29 to 43 percent,apparent desulfurization rate based on unit material flow density reach to Ks?0.0872kg·t-1,the service life of snorkels kept stable with the change put into effect.?3?Sufficient Ca Si cored wire was injected into the molten steel when RH process ended to modify inclusions of Al2O3 to be lower melting point 12CaO·7Al2O3inclusions,but the new type of inclusions always bring ultrasonic test defects in final10Cr Ni3Mo V plates for inclusion numbers with size over 10?m reached to 37.4/mm2on average.With moderate injection of Ca-Mg composite cored wire,inclusions of Ca O-Al2O3-MgO formed massively and amount of the inclusions decreased and the size was reduced compared with CaSi cored wire injection,but with excess Ca-Mg wire injection the size was apt to rise to over 8?m.In general,qualification rate of ultrasonic detection with Ca-Mg wire was apparently higher than that with CaSi wire.Ca S in inclusions changed relatively with different amount of wire injection.?4?The impact toughness and ductility of 10CrNi3Mo V plate was strong influenced obviously by amount,type and size of inclusions,samples from plate that with 1kg/t Ca-Mg wire injection had best impact toughness values?KV2?,that is room temperature to 309.2J and-40?to 295.2J on average,and the ductility were all over18%,this could be attributed to less inclusions in general and limited inclusions with size over 8?m.Technology integration including high efficiency top-bottom blowing,Ultra-low[P]and[S]liquid smelting,effective RH degassing and desulfurization and reasonable inclusion modification path was formed through systematic study on BOF-RH-CC metallurgical process for 10CrNi3MoV steel.Medium-alloy steel's high cleanness smelting,sustainable production and high performance supplying was realized and this also provides solid theoretical and practical foundation. |
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