Q355B铝镇静钢夹杂物演变及钙处理工艺优化

Based on the high melting point inclusions generated in the smelting process of Q355B aluminum calm steel in a steel plant, there is a problem of unqualified flaw detection. The evolution of inclusions in steel was analyzed by whole process sampling. It was found that the inclusions before calcium treatment in LF refining process were mainly CaO-MgO-Al₂O₃ inclusions with low CaO content, and the Al₂O₃ mass percent was about 77%. After calcium treatment, the CaO-MgO-Al₂O₃ inclusions in the molten steel are close to the region with high CaO content on the left side of the liquid phase region, and the Al₂O₃ mass fraction is reduced to 32%. At the same time, CaS heterogeneous nucleation on the surface of calcium aluminate, CaS-CaO-Al₂O₃ system inclusions appear, CaS content in inclusions increases to 23%. The thermodynamic equilibrium model was used to calculate the reaction equilibrium curves of S-Ca, Al-Ca and Al-S in calcium treated molten steel. Low-melting-point calcium aluminate inclusions such as C₃A, C₁₂A₇ and CA_L are generated at 1 873 K, and the relationship between w([Al]) and w([Ca]) in the molten steel should meet the following requirements:w([Al])²/w([Ca])³≤7.83×10³, 2.36×10⁵, 1.18×10⁷, and the relationship between w([Al]) and w([S]) should also be satisfied; w([S])³×w([Al])²≤7.79×10^-12, 8.36×10^-11, 8.14×10^-10; when w([Al]) in molten steel is 0.007 5%, w([Ca]) is controlled between 0.000 62% and 0.001 9%, and w([S]) is controlled between 0.001 6 % and 0.005 1%, which is conducive to the formation of ideal liquid product C₁₂A₇. With inclusion analysis and thermodynamic calculation, the production processes such as deoxidation and feeding line were optimized and adjusted. The addition amount of aluminum block was reduced from 0.8 kg/t to 0.7 kg/t, and the feeding line amount of silicon and calcium was reduced from 300 m/furnace to 200 m/furnace. The change of inclusion was analyzed through the whole process sampling. It was found that the mass percent of CaS in the inclusions of CaS-CaO-Al₂O₃ system decreased to about 5% after calcium treatment, and the inclusions were distributed near the low melting point liquid region. The molten steel w([Ca]) in the billet decreased from 0.003 1% to 0.001 5%-0.002 2%. The final inclusion system is (CaS)-CaO-(MgO)-Al₂O₃ low melting point composite inclusion, which prevents the formation of high melting point calcium aluminate inclusions and CaS inclusions and improves the quality of billet.