| Total oxygen content and inclusion in steel are the main factors which determinethe spring steel performance, reducing the total oxygen content and controllinginclusion plasticization have always been the key point and difficulty of spring steelsmelting, furnace refining is the key to smelt high quality spring steel For this, thisarticle took the Si-Mn deoxidization smelting spring steel60Si2MnA produced by adomestic factory as an example, studied the cleanliness of the spring steel.The totaloxygen content and the change of the non-metallic inclusions in the spring steel werealso analyzed, and optimized the thermodynamics in view of the compositions of theliquid spring steel, inclusions and refining slag. Secondly, according to the oftenproblems from inclusion control of the smelting of Al deoxidization combined withcalcium treatment ultra-low oxygen60Si2MnA spring steel, the effect of theconcentration of [Al],[Ca] and [S] in liquid steel and the liquid steel temperature onAl2O3inclusions transforming behaviour had been discussed by thermodynamicscalculation, as well as the thermodynamic analysis and control of calcium aluminate andsulfide precipitation in liquid steel.For the Si-Mn deoxidization smelting60Si2MnA spring steel, By sampling andanalyzing, we found that the (FeO+MnO) content in slag was2%~3%and the T.Ocontent is25~30ppm when the basicity of slag was about1;the number of the inclusionwas large, the number of the inclusion of per unit area was about38and the size wasapproximately8um;most of them was spherical brittle Al2O3and deformation(SiO2,MgO,CaS) inclusion, there were a large number of MnS inclusions, the silicateinclusions were not found.To smelt60Si2MnA spring steel by Si-Mn deoxidization, the thermodynamiccalculation shows that the lower the temperature the more conducive to precipitate lowmelting point inclusion at lower oxygen content in liquid steel;1600℃, when [Al]<24.5×10-6, Al2O3does not precipitate, when [Al]<24.5×10-6,1.38×10-11<a[Ca]<3.98× 10-10, CaO·Al2O3·2SiO2of low melting point can precipitate in steel, but thecontrollable scope of precipitating the inclusion is small;1479℃, when [Al]<12×10-6,2.19×10-12<a[Ca]<5.62×10-11, CaO·Al2O3·2SiO2can precipitate in steel, thecontrollable scope is smaller; when the w(Al2O3) in slag is0%~20%and the basicity isabout0.8, inclusions of low melting point can precipitate in liquid steel; MnS can notprecipitate during refining process, neither in solidification process, but the sulfurconcentration is larger when molten steel start to solidify, MnS tends to precipitate.To smelt ultra-low oxygen60Si2MnA spring steel by Al deoxidization, thethermodynamic calculation shows that1600℃,12(CaO)·7(Al2O3) will precipitate whenw[Ca]>0.0034%; when w[S]<0.005%, the critical calcium content significantlyincreases with the decreasing of sulfur content; when w(Al)=0.03%, the critical sulfurcontent of calcium aluminate and sulfide precipitation decreased to0.0037%,0.0008%and0.04%according to (CaO)·(Al2O3),12(CaO)·7(Al2O3) and3(CaO)·(Al2O3).Inorder to control the inclusions in low melting point12(CaO)·7(Al2O3) area withoutprecipitating calcium aluminate and sulfide inclusion, w[S] need to be controlled under0.0037%in the liquid steel; Before liquid steel solidification, control [S]<0.0004%,[S]will not react with calcium aluminate to generate the CaS during the solidificationprocess, but solute segregation in solidification process will happen, which leads to thelocal enrichment of [Ca],[S], and there will be a small amount of elemental CaSprecipitation.By thermodynamic calculation of the solubility of CaS in calcium aluminate,itshows that with the increasing of CaO content in inclusions, the distribution ratio ofsulfur and the solubility of calcium sulfide increases, the distribution ratio of sulfur is110.5in12(CaO)·7(Al2O3), the solubility of calcium sulfide is0.746%; In3(CaO)·(Al2O3), the distribution ratio of sulfur is478.3, the solubility of calcium sulfideobviously increases to3.23%. |
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