Q345D钢精炼过程夹杂物生成及演变行为

冶炼Q345D钢时由于夹杂物导致的探伤不合格情况时有发生,为了进一步去除和控制钢中非金属夹杂物,通过工业试验研究了"LF精炼→RH真空精炼→钙处理→软吹→连铸"工艺中的夹杂物生成及演变规律,并通过热力学计算优化钙处理工艺.结果 表明,转炉炉后及LF进站时采用铝强脱氧,夹杂物主要为Al2O3,LF精炼过程采用高碱度、强还...

Formation and evolution of inclusions in Q345D steel during secondary refining process

The unqualified flaw detection caused by inclusions occurs from time to time when smelting Q345D. In order to further remove and control non-metallic inclusions in steel. The formation and evolution of inclusions in Q345D refined by " LF refining→RH vacuum refining→calcium treatment→soft blowing→continuous casting" was studied through industrial experiments. The process of calcium treatment modified effectively by thermodynamic analysis. The results showed that the inclusions in liquid steel were consisted of Al₂O₃when aluminum was used in liquid steel after the converter and at the beginning of LF refining. Slags which have high alkalinity and strong reducibility are used in liquid steel during the LF refining. Liquid steel was strongly stirred during RH vacuum refining. Calcium was wired into the liquid steel after RH vacuum refining. The inclusions in liquid steel were consisted of CaO-MgO-Al₂O₃ ternary system and MgO-Al₂O₃binary system at the end of ladle furnace refining. The percentages of CaO-MgO-Al₂O₃ ternary system and MgO-Al₂O₃binary system were 67% and 33%, respectively. At the end of RH vacuum degassing, inclusions in liquid steel were consisted of CaO-MgO-Al₂O₃ ternary system, MgO-Al₂O₃binary system and CaO-Al₂O₃ binary system, and their percentages were 49%, 12% and 39%, respectively. After RH vacuum refined, the calcium wire was fed into liquid steel for calcium treatment. Average mass percent of CaS in inclusion was about 27% after calcium treatment. More CaS precipitated on the surface of calcium aluminate to generate an outer layer of CaS-CaO-Al₂O₃ ternary system. And the cores of CaO-MgO-Al₂O₃ ternary system were closer to the low-melting zone in the liquid steel. In the afterwards soft blowing and casting, mass percent of CaS in inclusions increased to 42%. In addition, the relationship between the quantitative density and the average size of inclusions during refining is conjugated. Quantitative density of inclusions increased from 2.32 pcs/mm2 after LF refining to 32.23 pcs/mm2 during continuous casting process. Average size of inclusions decreased from 2.88 μm after LF refining to 1.74 μm during continuous casting process. Thermodynamic calculations on inclusions at 1 873 K indicate that liquid inclusions can be more effectively modified, when the mass percent of Al and Ca in liquid steel is about 0.04% and in the range of 0.001 2%-0.002 8%, respectively.